Aspergillus infection was studied in patients admitted to the Bone Marrow Transplant (BMT) Service at the Johns Hopkins Oncology Center during a 9-year period. The overall incidence was 4% in 549 patients reviewed. The incidence at autopsy was 12% (21 of 174 patients autopsied). There was no difference in frequency of occurrence in allogeneic compared to autologous BMT recipients. However, all infections in autologous BMT patients (5 of 5) occurred during neutropenia before engraftment. In contrast, 16 of 17 infections in allogeneic BMT patients occurred after engraftment (p = 0.0002). This difference presumably related to differences in duration of neutropenia and immunodeficiency. Age, underlying disease, date of BMT, preparative regimen, remission status, prior treatment, interstitial pneumonitis and concomitant cytomegalovirus infection did not predispose patients to aspergillus infection. Different post-BMT immunosuppressive regimens did not affect the risk for aspergillus infection except that patients who were given cyclophosphamide plus methylprednisolone had a higher incidence of aspergillus infection than those given methotrexate (12% versus 1%, p = 0.03). Acute graft-versus-host disease imposed a slight risk for infection (p = 0.06).