We attempted to clarify the association between HLA and Crohn's disease. HLA-A, -B, -C and -DR locus antigens in 62 Japanese patients with Crohn's disease were analyzed and the results were compared with findings of 231 healthy Japanese. In the patients with Crohn's disease there was a strong association with HLA-DR4 and -DR5 (chi2 = 14 . 013, RR = 4 . 77 and chi2 = 9 . 345, RR = 5 . 04) and a weak association with HLA-Bw46 and -Bw51 (chi2 = 7 . 077, RR = 2 . 63, and chi2 = 5 . 401, RR = 2 . 52). There was a close association between HLA-DR5 in those with the ileocaecal type, and the -Bw51 and small intestine type. Therefore susceptibility to Crohn's disease may relate to specific allotypes on the human major histocompatibility complex. The correlation was weaker than the other diseases such as ankylosing spondylitis and Coeliac disease.