Fungi such as Fusarium species, Trichosporon species, Curvularia species, and Alternaria species previously were thought to represent contamination or harmless colonization when isolated from immunocompromised patients. More recently, the pathogenic role of these and other fungi has been clearly established. Three diverse groups of fungi are responsible for these emerging infections: the agents of phaeohyphomycosis and hyalohyphomycosis and certain yeasts. Reports of the emergence of these organisms as significant pathogens may be ascribed to increasing awareness by physicians and microbiologists, aggressive culture of patient specimens, increasingly cytotoxic chemotherapy, and selection of resistant organisms by the widespread empirical use of amphotericin B. Infections with these fungi tend to be disseminated and are frequently fatal in immunocompromised hosts. Treatment of these infections is not standardized. Experimental therapy in murine models of fungal infections suggests a role for newer agents, combination antifungal chemotherapy, and immunotherapy.