In recent years there has been an extensive search for associations between specific diseases and antigens of the major HLA histocompatibility complex. This search was stimulated by the demonstration of a genetic linkage between the mouse's major histocompatibility complex and resistance to viral-induced leukemias, specific autoimmune diseases, and specific immune responses. Recent studies in man have shown striking associations between HLA and rheumatic (ankylosing spondylitis), dermatologic (dermatitis herpetiformis, psoriasis), gastrointestinal (celiac disease), neurologic (myasthenia gravis, multiple sclerosis), and several other diseases. These associations are identified by the relative increases in the frequency of particular antigens in the groups of diseased patients as compared with racially and ethnically matched control groups. This control is important as the normal frequency of certain HLA types varies considerably in various parts of the world, eg, HLA-B5 is found in 34% of Japanese and in only 4% of Africans, and HLA-B8 is found in 25%