During September 1990-March 1991, clinicians at a New York City (NYC) hospital diagnosed Mycobacterium haemophilum infections in four patients. Two cases occurred in persons with acquired immunodeficiency syndrome (AIDS), and two occurred in persons who had received bone marrow transplants and were on therapy to prevent a graft-versus-host reaction. In April 1991, three additional M. haemophilum infections in persons with AIDS were identified in the NYC area. Because M. haemophilum infection rarely has been recognized, intensive case finding was initiated in NYC as a result of these reports.