The common occurrence of a type of monarticular arthritis that frequently simulates tuberculosis very closely is indicated by a study of a series of cases in which the patients were treated at the New York Orthopaedic Dispensary and Hospital and the results of which are presented in this paper. Few references to the condition appear in the literature. Friedrich1 and Spitzy2 mentioned that monarticular arthritis may be mistaken for tuberculosis and that the differential diagnosis is often difficult. A conviction, which has become more firmly established with added experience, that in many instances of chronic diseases of the joints a diagnosis cannot be made without exploratory operation, biopsy and tests on guinea-pigs3 has led to the discovery of this group of cases, which presents a number of interesting features. From March, 1924, to January, 1930, twenty-four patients with such cases were operated on in the New York