Nine patients with lymphoma occurring in association with inflammatory bowel disease were admitted to The Mount Sinai Hospital between 1960 and 1983. Five (two men and three women) occurred among 1156 patients (0.43%) with ulcerative colitis (UC) and four (men), among 1480 patients (0.27%) with Crohn's disease (CD), a strong male preponderance in the latter group. In all four of the patients with CD and in four of the five patients with UC, the lymphomas were extraintestinal. The mean age of onset of UC in these patients was late (46 years, 19 years older than in our overall series), with lymphomas occurring a mean of only 12 years later. By contrast, patients with CD had bowel disease much younger (mean age, 26 years), and their lymphomas appeared after a longer disease duration (mean, 24 years). The risk factors for the one patient with colonic lymphoma were similar to those with colitis-associated colorectal carcinoma: extensive and long-standing colitis and relatively young age when malignant disease developed. Four of the patients with lymphoma had associated colonic carcinoma; in three of them, the carcinoma appeared within the first decade of colitis, an unusual occurrence. A second malignant lesion also occurred in three patients with UC.