To test the hypothesis that psoriasis is associated with Crohn's disease and ulcerative colitis, 204 patients with inflammatory bowel disease (116 with Crohn's disease and 88 with ulcerative colitis) and 204 age and sex matched controls were interviewed and examined. The prevalance of psoriasis in Crohn's disease (II.2%) and in ulcerative colitis (5.7%), was significantly greater than in the control group (1.5%). The prevalence of psoriasis in first degree relatives of patients with inflammatory bowel disease was also increased. It is suggested that there is a relationship between psoriasis, ankylosing spondylitis, sacroiliitis, peripheral arthropathy and inflammatory bowel disease, which may be explained by common genetic factors.