Ninety-one patients with documented invasive infections due to an Aspergillus species were identified at Memorial Sloan-Kettering Cancer Center from July 1, 1971, through December 31, 1976. Of the 29 patients in whom the diagnosis was made during life, 10 had successful treatment and survived the Aspergillus infection by two to 17 months. An immunodiffusion test was useful in the early diagnosis of invasive aspergillosis, and in 11 patients in whom the diagnosis was supported by seroconversion and who underwent treatment, the survival rate was 64 percent. Cultures of respiratory secretions were not reliable because they often reflected only colonization. In one year, only 9 percent of he patients with Aspergillus species isolated from the sputum had an invasive infection. The lung was the commonest site of involvement, 91 percent of the patients having pulmonary lesions. The most frequently affected extrapulmonary organ was the brain (18.3 percent). Eight patients had nonpulmonary aspergillosis as the only manifestation of this infection. Most of the 91 patients had hematologic neoplasms as the underlying disease, and neutropenia and antibacterial therapy preceded the diagnosis of aspergillosis in the majority of cases.