Your bladder is a muscular, balloon-shaped organ located in your pelvis. It stores urine that your kidneys produce during the process of filtering your blood. Like a balloon, the bladder can get larger or smaller depending on the amount of urine it holds. Urine passes from each kidney into your bladder through a thin tube called a ureter and is eliminated from your body through another narrow tube, the urethra. A woman's urethra is relatively short, whereas a man's is longer, passing through the prostate gland and ending at the tip of the penis.

Healthy cells grow and divide in an orderly way. This process is controlled by DNA — the genetic material that contains the instructions for every chemical process in your body. When DNA is damaged, changes occur in these instructions. One result is that cells may begin to grow out of control and eventually form a tumor, a mass of malignant cells.

Most bladder cancers begin in the specialized cells that line the walls of your bladder (transitional cells). The same type of cells occur in your kidneys, ureters and urethra where they may also give rise to malignant tumors. When chronically exposed to certain chemicals, especially those in tobacco smoke and certain dyes, these cells can become cancerous.

Some cancers remain confined to the bladder lining (carcinoma in situ — in one place). But other cancers are invasive, growing into or through the bladder wall, and eventually into nearby lymph nodes and adjacent organs. In time, cancer may spread (metastasize) to other organs, including the lungs, liver or bones.

Just what causes bladder cancer isn't entirely clear. Certain inherited metabolic factors may play a role. People whose bodies metabolize toxic chemicals quickly may be less susceptible to bladder cancer than people who metabolize the same chemicals more slowly. Chief among these chemicals are polycyclic hydrocarbons found in cigarettes and in some industrial chemicals.

Cigarettes, in fact, are the leading known cause of bladder cancer. That's because cancer-causing chemicals (carcinogens) in tobacco can become concentrated in your urine and eventually damage the lining of your bladder.

Smokers are two to three times more likely to develop bladder cancer than are nonsmokers. The risk is even greater for women. Cigarette for cigarette, women have twice the risk of bladder cancer that men do.

Other known causes of bladder cancer include:

• Occupational exposure to carcinogenic chemicals. This is especially true of chemicals used in the oil, rubber, dye, paint and textile industries. Experts estimate that at least 25 percent of all male bladder cancers result from occupational exposure, even if that exposure was brief. Often, these cancers develop many years later. Smokers who work with toxic chemicals are at especially high risk of bladder cancer.

• Exposure to arsenic. People living in areas where pesticides are widely used and the drinking water contains high levels of arsenic are more likely to develop bladder cancer. Although exactly what constitutes dangerous levels of arsenic remains controversial, the risk of developing lung and bladder cancer from arsenic appears higher than originally thought.

• Treatment with certain drugs. Treatment with the anti-cancer drugs cyclophosphamide (Cytoxan) and ifosfamide (IFEX) increases your risk of bladder cancer. If you're treated with these drugs, your doctor is likely to prescribe another medication, mesna, to help reduce bladder irritation.

• Infection with parasites. In some parts of the developing world, especially Egypt, a chronic parasitic infection (schistosomiasis) can lead to a type of bladder cancer called squamous cell carcinoma. Both the infection and squamous cell carcinoma are rare.

• Use of the herb Aristolochia fangchi. This Chinese herb, which is included in some weight loss supplements, has been linked to both bladder cancer and kidney failure. A number of other factors have been suggested as possible causes of bladder cancer, including:

• Chlorine by-products. These form when chlorine is added to drinking water. Although there's no definitive proof of a link between chlorine by-products and bladder cancer, research in this area is ongoing.

• Cyclamate. Early studies linking this artificial sweetener to bladder cancer in mice caused the Food and Drug Administration (FDA) to ban cyclamate in 1969. Subsequent studies haven't found as clear a connection.

• Saccharin. Animal studies have shown a relationship between this artificial sweetener and bladder cancer. But the connection between saccharin and cancer in humans isn't clear. For now, products containing saccharin carry a warning about a possible link to bladder cancer.

If you think you're at risk of bladder cancer, discuss your concerns with your doctor. He or she may be able to suggest ways to reduce your risk. Also, keep in mind that having one or more risk factors doesn't mean you'll develop the disease. In general, the following factors may increase the likelihood that you'll develop bladder cancer:

• Smoking. Smoking is the single greatest known cause of bladder cancer. The risk increases with the number of cigarettes smoked a day and the number of years you've smoked.

• Industrial chemicals. Repeated exposure to chemicals used in the manufacture of dyes, rubber, leather, textiles and paint products may increase your risk of developing bladder cancer years later.

• Age. The chance of getting bladder cancer increases as you grow older. The average age at diagnosis is 68 or 69. People younger than 40 rarely get the disease.

• Race. Caucasians are twice as likely to develop bladder cancer as blacks and Hispanics are. Asians have the lowest rates of the disease.

• Sex. Men are two to three times more likely to get bladder cancer than are women.

• Certain drugs. Treatment with certain anti-cancer drugs increases your risk of bladder cancer.

• Chronic bladder inflammation. Chronic or repeated urinary infections or inflammations may increase your risk of a certain form of bladder cancer. But doctors don't believe infection or inflammation alone causes cancer.

• Family history. You're at higher risk of bladder cancer if you have family members with the disease.

• Personal history. Having bladder cancer once makes it more likely you'll get it again. Tumors may recur in your ureters or urethra as well as in the bladder itself.