We reviewed the records of 32 patients with acute leukemia and proved invasive fungal infections to determine the clinical and pathologic characteristics of systemic mycosis in patients undergoing intensive induction chemotherapy. The incidence of invasive fungal infections among our patients was at least 27 percent, and Candida and Aspergillus accounted for the majority of these infections. Patients with systemic candidiasis generally had prolonged severe neutropenia, fever refractory to antibiotics, and evidence of mucosal colonization by fungi. At autopsy, Candida was always widely disseminated. Patients with aspergillosis generally had neutropenia, fever, and pulmonary infiltrates at the time of admission to the hospital and, at autopsy, their infections were primarily confined to the lungs. Patients infected with both Candida and Aspergillus had clinical and pathologic findings that were a combination of the features of each type of infection. A diagnosis of invasive fungal infection was established before death in only nine of the patients, all of whom had systemic candidiasis. Four of these patients were successfully treated and survived their hospitalization. The reasons for frequently misdiagnosing and unsuccessfully treating systemic mycosis in patients with acute leukemia are examined, and suggestions are made for improved management of patients at high risk for these infections. These suggestions are based upon recognition of the clinical settings in which fungal infections occur, the aggressive use of invasive diagnostic procedures, and the early empiric use of amphotericin B.