Penicillium marneffei is a dimorphic fungus that causes one of the most common opportunistic infections in Asian patients with AIDS. The diagnosis is established by microbiologic culture, requiring at least several days. A presumptive diagnosis can be made by cytologic or histologic examination of biopsied or aspirated tissue. Bone marrow biopsy is often performed in the workup of patients with AIDS who have fever or hematologic abnormalities and can provide prompt diagnosis of opportunistic infection. We report the bone marrow findings in the largest series of patients with culture-proven P marneffei infection. In the bone marrow, the histiocytes can occur in large numbers and be easily recognized or may be extremely subtle. P marneffei infection is sometimes not accompanied by granuloma formation despite marked histiocytic proliferation. The histiocytes contain a few to many intracellular yeast-form cells that resemble cellular debris because of their small size and staining pattern. The characteristic septate forms and the absence of budding help distinguish the condition from histoplasmosis and toxoplasmosis. Routine performance of silver methenamine stain for fungi in marrow biopsy specimens of febrile patients with AIDS is recommended to detect a subtle infection.