We retrospectively assessed the clinical course in four patients with long-standing Crohn's disease who became infected with human immunodeficiency virus (HIV). The duration of active Crohn's disease was 21, 10, 4, and 4 years in our four patients. They experienced a stable remission of Crohn's disease symptoms after HIV infection. In three patients Crohn's disease was in stable remission for 5, 8, and 8 years after HIV infection and all three died from acquired immunodeficiency syndrome-related disease. One patient was still alive without recurrence of Crohn's disease symptoms 7 years following HIV detection. Our observations of a spontaneous improvement in the clinical course of Crohn's disease after HIV infection, suggests that the integrity of the immune response, especially that of CD4 T cells, plays a major role in the tissue injury mechanism in Crohn's disease.