In general, tumors appear to be caused by abnormal regulation of cell division. Typically, the division of cells in the body is strictly controlled -- new cells are created to replace older ones or to perform new functions. Cells which are damaged or no longer needed die to make room for healthy replacements.

If the balance of cell division and death is disturbed, a tumor may form. Tumors are classified as either benign (slow-growing and often harmless depending on the location) or malignant (faster-growing and likely to spread to other parts of the body and cause problems). Malignant tumors are what we call cancer.

Abnormalities of the immune system, which usually detects and blocks aberrant growth, can lead to tumors. Other causes include radiation, genetic abnormalities, certain viruses, sunlight, tobacco, benzene, certain poisonous mushrooms, and aflatoxins (a poison produced by an organism which sometimes grows on peanut plants). Tobacco causes more deaths from cancer than any other environmental agent.

Some tumors are more common in one sex than the other, some are more common among children or the elderly, and some vary with diet, environment, and genetic risk factors.