Trichosporonosis due to Trichosporon beigelii or T. capitatum is an infrequent but potentially fatal invasive fungal infection in cancer patients. We studied epidemiologic, clinical, pathologic, and microbiologic features of this infection during a 7-year period at the University of Maryland Cancer Center. Fifteen patients with involvement by Trichosporon were identified: 5 were infected, 5 were possibly infected, and 5 were colonized but not infected by Trichosporon. Four of the infected patients had trichosporonemia and/or positive skin biopsy cultures as the first evidence of infection. The fifth infected patient had positive marrow and skin biopsy cultures. Serial surveillance cultures of infected patients showed preceding Trichosporon colonization in only 1 of 5 cases. Pulmonary infiltrates in 3 infected patients correlated at postmortem examination with Trichosporon pneumonia. Renal dysfunction marked by proteinuria, hematuria, red blood cell casts and azotemia correlated with widespread glomerular infiltration with the fungus. The five infected patients died of their infection, whereas the 2 possibly infected patients who died succumbed to their underlying illness. Trichosporonemia may have been averted in possibly infected patients because of a shorter median duration of profound (less than 100/microliter) neutropenia (5 days) when compared to that of infected patients (20 days). No environmental source of Trichosporon was found in environmental surveillance cultures of food, air, or inanimate surfaces. In vitro studies of three pathogenic strains showed resistance to 5-fluorocytosine but susceptibility to amphotericin B, ketoconazole, and miconazole. Norfloxacin augmented the in-vitro antifungal activity of amphotericin B. Trichosporon must be considered an opportunistic pathogen that can cause serious infections among patients with cancer.