Five hundred consecutive admissions to the rheumatism service of a large hospital were reviewed in an effort to establish criteria for accurate differential diagnosis in rheumatic disorders. Forty-one per cent of the patients presented evidence of articular involvement or arthritis; 28 per cent had non-articular rheumatism, embracing the various types of fibrositis; 7 per cent had musculoskeletal neuroses, and 24 per cent had a variety of diseases unrelated to the musculoskeletal system. A working classification of the various causes of musculoskeletal pain is presented and criteria for the differentiation of the individual diseases are suggested.