Mycobacterium haemophilum, previously characterized as an unusual pathogen, is found primarily in immunocompromised hosts. This organism has stringent growth characteristics and may not be isolated using routine techniques. M. haemophilum infects the skin and underlying tissues, a circumstance which reflects the organism's propensity for growth in a cooler environment. Infections have been reported in renal transplant recipients, patients with Hodgkin's disease, and, more recently, patients with AIDS. The organism has also been isolated from children with cervical lymphadenitis in the absence of apparent immunodeficiency. Response to therapy has not been uniform, and in some instances improvement in immune status has been associated with regression of lesions. With proliferation of transplantation surgery, chemotherapy, and AIDS, the number of infections due to M. haemophilum is likely to increase.