Currently, about 180 hospitals participate in the National Nosocomial Infections Surveillance (NNIS) system. From January 1980 through April 1990, 27,200 fungal isolates causing nosocomial infections were reported from these hospitals; Candida species accounted for 19,621 (72.1%) of these isolates. Immunocompromised patients are at particularly high risk for candidemia. In patients with acute lymphocytic leukemia, treatment with vancomycin and/or imipenem appears to be an independent risk factor for candidemia; colonization of stool by Candida species may be another important predisposing factor in these patients. Rapid detection of invasive candidemia in these high-risk patients is particularly important to the improvement of rates of survival. Methods for rapid detection, such as the measurement of mannan (the major cell-wall polysaccharide of Candida), may be useful for diagnosing invasive candidiasis and for monitoring the response of this infection to antifungal therapy. Further studies of risk factors and the development of new methods for rapid diagnosis and monitoring should help decrease the morbidity and mortality associated with nosocomial fungal infections.