A 29-year-old man from northern Thailand had a three-week history of recurrent fever (a temperature of up to 40°C), generalized weakness, poor appetite, and weight loss. One week before hospitalization, papular skin lesions developed on his face (Panel A), chest, and arms, which enlarged and became umbilicated. He was found to be positive for the human immunodeficiency virus on enzyme-linked immunoassay but had not previously had an acquired immunodeficiency syndrome–defining illness. He was treated for a disseminated Penicillium marneffei infection with intravenous amphotericin B for two weeks, and the fever and skin lesions resolved. Therapy with oral itraconazole was then given for eight weeks. A bone marrow aspirate (Panel B) showed numerous bipolar intracellular and extracellular basophilic yeast-like organisms (Wright's stain, ×400). Some showed clear septation as they were undergoing division. Division by binary fission (arrows in Panel B) is characteristic of P. marneffei and is useful in differentiating the organism from Histoplasma capsulatum, which multiplies by budding. P. marneffei can readily be cultured from blood, bone marrow aspirates, and biopsy specimens of skin or lymph nodes. The mycelial form of the organism was isolated by incubating the patient's bone marrow aspirate on Sabouraud's dextrose agar for five days at 25°C. A soluble red pigment was produced that diffused into the medium (Panel C). Before concluding that the isolate is P. marneffei, it is also necessary to show that when grown at 37°C on 5 percent sheep's-blood agar, penicillium organisms are converted to a yeast-like form.