We report a child with cartilage-hair hypoplasia and Hodgkin's disease who developed progressive renal failure and died following infection with a polyomavirus, BK virus. Renal biopsy showed interstitial inflammation, tubular atrophy, and intranuclear inclusions in tubular epithelium, with large numbers of papovavirus particles within the cells. BK virus infection was confirmed by polymerase chain reaction on renal biopsy material and in urine and the demonstration of a high titre of haemagglutination inhibition (HI) and IgM antibody to the virus in the patient's serum. This case emphasises the need to study in depth any unusual clinical manifestation in immunocompromised patients to delineate better the full clinical impact of less well-established pathogens such as BK virus.