Data on associations between antigens of the major human histocompatibility system, HL-A, and various non-malignant diseases have been collected from the literature available at the beginning of April, 1974. Combined estimates of relative risks of all diseases studied were calculated for the antigens showing the strongest associations. Definite associations between HL-A antigens and disease exist for the following conditions: ankylosing spondylitis, Reiter's disease, reactive arthritis, acute anterior uveitis with W27, psoriasis vulgaris with HL-A13 and 17, coeliac disease, dermatitis herpetiformis, auto-immune aggressive hepatitis, myasthenia gravis with HL-A8; insulin-dependent diabetes with HL-A8 and W15, and multiple sclerosis with HL-A7. Probable associations were found between psoriatic arthritis and W27, and Grave's disease and HL-A8. The relative risks are almost invariably above one (indicating susceptibility) and for the definite associations they vary from 1.4 (multiple sclerosis in HL-A7 positives) to 120.9 (ankylosing spondylitis in W27-positives).