The incidence of adenovirus (AV) infections following SCT was determined in a prospective multicenter trial. Over 1 year, 130 consecutive patients undergoing allogeneic SCT at Essen University Hospital were included and followed for 6 months. Source of stem cells was blood in 68 cases. Fifty-eight patients had HLA-identical sibling donors. Throat swabs, urine and stool samples were screened weekly for AV antigen and DNA by ELISA and nested PCR, respectively. In 35 cases adenovirus infection was detected. There was no seasonal variation. Throat swabs were positive in 24, urine in 12, and stool in 11 cases, resulting in a cumulative risk of infection of 29%. The incidences of AV infection of the respiratory, gastrointestinal and urinary tract were 19%, 10%, and 9%, respectively, and infections were diagnosed after a median (range) interval of 44 (−2–179), 37 (−2–168), and 53 (17–153) days after transplantation. On multivariate analysis, presence of AV antibody in the donor and acute graft-versus-host disease grade IV were found to be independent risk factors for AV infection. Eleven patients had AV isolated from more than one site and five patients had probable AV disease. We were not able to identify patients in whom AV infection was the leading cause of death. The majority of patients infected with AV suffered from severe acute graft-versus-host disease often accompanied by other opportunistic infections, such as aspergillosis or CMV reactivation. Nineteen out of 36 patients who died during the observation period had AV infection. In summary, AV infection after allogeneic SCT was observed in a substantial number of patients. In addition to well-known risk factors for viral infection after SCT we were able to demonstrate that a positive AV antibody test in the donor is an important risk factor for AV infection. Further studies are needed, however, before final conclusions on the clinical sequelae of AV infection can be made and the role of preventive and therapeutic strategies toward AV infection after allogeneic SCT can be defined. Bone Marrow Transplantation (2001) 28, 51–57.