The case histories of 1961 patients with inflammatory bowel disease (IBD), 1227 with Crohn's disease (CD) and 734 with ulcerative colitis (UC), have been studied for the incidence of extraintestinal malignant neoplasms. There were 54 extraintestinal cancers in 51 patients: 28 patients with CD and 23 with UC; 25 men and 26 women. There were 9 breast, 7 skin, 15 reticuloendothelial, 11 genitourinary, 3 lung, 3 perianal, 2 pancreatic islet cell, and several miscellaneous cancers. The number of patient-years from the onset of disease to the last date of follow-up was calculated for men and women with each form of IBD. The observed number (O) of neoplasms was recorded. The expected number (E) of neoplasms was derived from the Department of Health, Education, and Welfare (DHEW) incidence figures for the same neoplasms that occurred in a standard age- and sex-matched population. The O/E ratio was then calculated for each type of cancer as well as for the entire series. There were no statistically significant increases in overall O/E ratios of extraintestinal cancers for either CD (0.76) or UC (1.32). On the other hand, several specific types of cancer did appear to occur with a frequency that was significantly greater than expected. These cancers were classified into two groups. The first group included reticuloendothelial neoplasms. There was an excess of leukemias in UC (P less than 0.005) and an excess of lymphomas in both UC and CD (P less than 0.005). The second group included three squamous cell cancers of the perianal region, an incidence 30 times greater than expected, and two squamous cell cancers of the vagina, also in excess of the expected number. Lymphoma, leukemia, and squamous cell cancers have been reported to occur in excess in immunosuppressed or irradiated patients. It may therefore be speculated that the apparently increased incidence of these neoplasms in the patients with ileitis and colitis might be related to immunologic deficiencies associated with IBD, to the long-term administration of steroids or other immunosuppressive medications that were given to most of the patients or, possibly, to increased exposure to ionizing radiation. The apparently increased incidence of perianal and vaginal cancers of the squamous variety might be a consequence of the combined effects of chronic inflammatory disease involving these areas and primary immune suppression.