BK papova virus infection is frequently seen after bone marrow transplantation as a causative agent of hemorrhagic cystitis. We report an 8-month-old child with osteopetrosis who died of a severe interstitial pneumonia after receiving an unrelated umbilical cord transplant. On autopsy, BK virus was detected in the lung tissue using immunofluorescence assay, cell culture and PCR. No other pathogens were recovered. BK virus infection should be considered as a cause of interstitial pneumonia in children undergoing transplantation.