Abstract
BK virus (BKV) is a human polyomavirus. The primary infection occurs typically without specific signs or symptoms. Almost 80% of adults are seropositive. Clinically relevant infections are usually limited to individuals who are immunosuppressed. After primary infection, BKV remains latent in the kidneys and can be reactivated in the setting of immunosuppression. BKV is associated with tubulointerstitial nephritis and ureteric stenosis in renal transplant recipients. Furthermore, BKV-induced haemorrhagic cystitis (HC) is a severe complication of bone marrow transplantation (BMT) in children and adults. A combination of HC and tubulointerstitial nephritis in a patient has not been reported so far. We report on an 11-year-old boy with acute myeloid leukaemia undergoing BMT. BKV infection was reactivated during post-transplant immunosuppressive therapy causing HC associated with tubulointerstitial nephritis.