The rate of isolation of mycobacteria other than Mycobacterium tuberculosis (MOTT) has increased over the past several years; in some areas the isolation rate for Mycobacterium avium-Mycobacterium intracellulare has surpassed that for M. tuberculosis. Simultaneously, the spectrum of clinical manifestations with the various species has widened. Outbreaks of disease due to Mycobacterium chelonae have occurred in renal dialysis patients. New species have been described: Mycobacterium malmoense is primarily a pulmonary pathogen, and Mycobacterium haemophilum has been recovered from cutaneous lesions in immunosuppressed patients. In addition, reports of disease due to species generally considered saprophytic have become more numerous. In this review, the epidemiologic, pathologic, and clinical features of the individual MOTT species are discussed. A brief summary of mycobacteria isolated at the Cleveland Clinic foundation between 1982 and 1985 is also presented.