During a 31-month period in 1979-1981, nine patients at a renal transplant center in Tennessee developed invasive infections with Aspergillus species. Despite an extensive search, no common environmental source of contamination was found. A matched case-control study of host risk factors showed that leukopenia, prior administration of antibiotics, and treatment with azathioprine and antilymphocyte serum were not significantly related to the development of aspergillosis. In contrast, the administration of high-dose corticosteroids posed a significant risk. An average daily dose of greater than or equal to 1.25 mg of prednisone/kg per day for the entire interval studied was the best predictor of subsequent invasive infection with Aspergillus.