To investigate the incidence, risk factors, and outcome of Aspergillus infections among marrow transplant recipients, records from 2496 patients were reviewed, and 214 patients had Aspergillus organisms identified. Of these, 158 had invasive aspergillosis, 44 were colonized, and 12 had contaminated cultures. The incidence of invasive aspergillosis increased from 5.7% to 11.2% during the study. The onset of infection was bimodal, peaking 16 and 96 days after transplant. For patients within 40 days after transplant, underlying disease, donor type, season, and transplant outside of laminar air flow rooms were associated with significant risk for invasive aspergillosis. For patients >40 days after transplant, age, underlying disease, donor type, graft-versus-host disease, neutropenia, and corticosteroid use were associated with increased risk of aspergillosis. Only 31% of infected patients were neutropenic at the time of diagnosis. The risk factors for aspergillosis depend on the time after marrow transplant and include both host and environmental characteristics.