In a review of a large number of patients with inflammatory bowel disease, leukemia was observed in five patients with chronic ulcerative colitis and in two patients with Crohn's disease. In ulcerative colitis patients, there were three cases of acute myelocytic leukemia and one case each of acute lymphoblastic leukemia and chronic granulocytic leukemia. In Crohn's disease patients, there was one case each of chronic granulocytic leukemia and chronic lymphocytic leukemia associated with thrombocythemia. Sixteen other cases of leukemia have been reported to date in inflammatory bowel disease. All types of leukemia, but particularly acute myelocytic leukemia, have been described. There has been no single common feature as to type (whether ulcerative colitis or Crohn's disease), extent and course, or medical and surgical treatment of the bowel disease. The relative risk of leukemia in patients with ulcerative colitis was 5.3 [95% confidence interval 1.7 to 12.3 (P less than 0.01)] and of acute myelocytic leukemia 11.4 [95% confidence interval 2.3 to 24.9 (P less than 0.01)]. Our data on patients with Crohn's disease were not sufficient to assess the statistical significance of leukemia in this disease. This study suggests that there may be an increased risk of leukemia, particularly acute myelocytic leukemia, in ulcerative colitis. The causal relationship, if any, remains undetermined.