We examined fertility in men with inflammatory bowel disease (IBD) to determine whether the reported deficit in pregnancies in their spouses was due to reduced reproductive capacity. The cases were 106 men with Crohn's disease and 62 with ulcerative colitis drawn from the rosters of North Carolina chapters of the National Foundation for Ileitis and Colitis. The 140 normal controls were neighbors identified by the cases. There were 186 pregnancies among the spouses of men with Crohn's disease, 110 among the ulcerative colitis group, and 300 among the controls. The mean number of pregnancies among the Crohn's disease patients (1.75 +/- 0.12) was significantly lower than the number for controls (2.14 +/- 0.11), p less than 0.02. The reduction in the mean number of pregnancies among patients with ulcerative colitis (1.77 +/- 0.18) was not significantly different from controls (p less than 0.07). Analysis of waiting time to conception showed that fecundability (the probability of pregnancy in a menstrual cycle) was not different in spouses of IBD patients and controls. We conclude that the overall reproductive capacity of men with IBD is not markedly diminished.