A clinical, radiological and immunogenetical study was carried out on 51 Crohn's patients. Rheumatological disorders were found in 16 of them, with higher frequency in those with colon involvement only. A statistically significant increase in the frequencies of HLA-A9 and HLA-Cw3 was noted: Cw3 showed a particularly high frequency in males, and A9 in younger patients. The frequency of HLA-B27 was significantly increased in the patients with colon involvement. In the group of 16 patients with rheumatic diseases HLA antigen frequencies were not significantly different from the control population.