From June 1990 to August 1991, 21 patients infected with the human immunodeficiency virus (HIV) presented with systemic mycosis caused by Penicillium marneffei. Between August 1987 and August 1991, only five patients were observed who had P. marneffei infection but not HIV infection. The clinical presentation included fever, cough, and generalized papular skin lesions. For 11 of these 21 patients, the presumptive diagnosis of P. marneffei infection could be made by microscopic examination of Wright's-stained bone marrow aspirate and/or touch smears of skin specimens obtained by biopsy several days before the results of culture were available. Initial clinical response to treatment with either parenteral amphotericin B or oral itraconazole was favorable in most patients. Epidemiological and clinical evidence suggest that this systemic mycosis is caused by an important opportunistic pathogen and that it should be included in the differential diagnosis of AIDS, at least for countries in areas of endemicity, i.e., Southeast Asia and China.