Haemorrhagic cystitis (HC) is an important complication after bone marrow transplantation (BMT). Overt HC (grade > or =2, gross haematuria, clot retention and impairment of renal function), clinically more important than mild and occult HC (grade 1, microscopic haematuria), leads to substantial morbidity and occasional mortality. We retrospectively analyzed 32 cases of clinically overt HC from a series of 236 BMT patients. Significant risk factors included the use of busulphan during conditioning, allogeneic BMT and acute GVHD. Logistic regression showed GVHD to be the most important risk factor. According to the time of engraftment, HC could be divided into pre- and post-engraftment subtypes. Pre-engraftment HC was brief, not more severe than grade 2, and subsided with supportive treatment. In contrast, post-engraftment HC was protracted, often of grade > or =3, associated with severe GVHD, and required surgical intervention in many cases. Polyoma BK viruria, but not adenoviruria, could be demonstrated in both types of HC. The increased severity and association with GVHD of post-engraftment HC suggested that attack of urothelium by immunocompetent cells, possibly directed against BK viral antigens, might play a pathogenetic role.