Late onset haemorrhagic cystitis (HC) occurs in 20-30% of allogeneic bone marrow transplant patients. Human polyomavirus BK (BKV) (or less frequently adenovirus) may be involved in the pathogenesis of viral HC and can represent a serious post-transplant complication. Diagnosis and treatment of viral HC can be difficult and has an uncertain outcome. We report the efficacy of sequential vidarabine in the treatment of a patient with severe BKV-associated HC, despite the delay in implementing therapy. Bone Marrow Transplantation (2000) 25, 319-320.