A prospective study of patients hospitalized in a large Veterans Administration Hospital between November 1963 and November 1973 revealed 123 patients with deep mycotic infections. The incidence of these infections almost doubled during the last 5 years. Candida (55 patients) and Aspergillus (26 patients) were the major causative agents. Nine other fungal caused infection in the remaining patients. Candidemia was rare prior to the introduction of commerical percutaneously-inserted venous catheters in 1965. The incidence increased further following the introduction of parenteral hyperalimentation in 1969, and Torulopsis fungemia (5 patients) appeared for the first time. Invasive pneumonia caused by spore-forming Aspergillus decreased when patients were moved from an old, naturally-ventilated hospital to a new, mechanically-ventilated one. The air in both hospitals was sampled on one occasion for the presence of fungal spores, and spores of Aspergillus fumigatus were detected only in the old hospital. Our experience suggests that hospital-acquired Aspergillus infection of the lung might be eliminated if all incoming hospital air is filtered, properly vented, and not recirculated. Efforts to decrease hospital-acquired fungal infections include vigorous infection control procedures for intravenous therapy, judicious use of any therapy that predisposes to infection, and further evaluation of improved mechanical control of hospital ventilation.