Invasive aspergillosis (IA) is rare among renal transplant recipients (RTRs). We investigated a cluster of IA among RTRs at a California hospital from January to February 2001, when construction was ongoing. We conducted a cohort study among RTRs who were hospitalized between January 1 and February 5, 2001, to determine risk factors for IA. IA was defined using established guidelines. Four IA cases occurred among 40 RTRs hospitalized during the study period. Factors associated with an increased risk of IA included prolonged hemodialysis, lengthy corticosteroid treatment posttransplant, and use of sirolimus alone or with mycophenolate (P<0.05). After the study period, three additional RTRs developed IA; two Aspergillus isolates recovered from these patients had indistinguishable profiles by DNA fingerprinting, suggesting common-source exposure. This study suggests that immunosuppressed RTRs can be at an increased risk for IA. Measures to prevent IA in these patients should be taken during hospital construction.