Eighty-six patients with laboratory evidence of human immunodeficiency virus infection presented to Chiang Mai University Hospital in Chiang Mai, Thailand, between 1 June 1990 and 30 June 1992 with systemic infection caused by the dimorphic fungus Penicillium marneffei. Thirty isolates of P. marneffei from clinical specimens from these patients were tested for their in vitro susceptibilities to amphotericin B, 5-fluorocytosine, miconazole, ketoconazole, itraconazole, and fluconazole. P. marneffei was highly susceptible to miconazole, itraconazole, ketoconazole, and 5-fluorocytosine. Amphotericin B showed intermediate antifungal activity, while fluconazole was the least active; some strains of the fungus were resistant to fluconazole. The clinical and microbiological responses correlated with the overall patterns of in vitro susceptibility to the azoles, whereas results with amphotericin B were more difficult to assess. Antibiotic failures of initial therapy occurred in 8 of 35 (22.8%) patients treated with amphotericin B, 3 of 12 (25%) patients treated with itraconazole, and 7 of 11 (63.6%) patients treated with fluconazole. Itraconazole or ketoconazole should be considered to be the drug of first choice in the treatment of mild to moderately severe P. marneffei infection. Parenteral therapy with amphotericin B may be required for seriously ill patients. Since at least 12 patients who responded to initial therapy relapsed within 6 months regardless of initial antifungal therapy, maintenance oral therapy with itraconazole or ketoconazole may be necessary.