Haemorrhagic cystitis (HC) is the syndrome of haematuria and symptoms of lower urinary tract irritability in the absence of bacterial infection. We report a low incidence of HC (18.2%) in 681 haemopoietic stem cell transplant patients, using a prophylactic regimen of hyperhydration and forced diuresis. The incidence of grade 3-4 disease is 3.4%. There was a marked difference in incidence between allogeneic and autologous transplant populations, 24.2% vs. 3.5% (P<0.0005). Busulphan conditioning, acute GVHD, interstitial pneumonitis and use of methotrexate and cyclosporin immune suppression were associated with significantly increased incidence of HC in the allogeneic population. This may reflect the numerous factors that contribute to the greater immunosuppression and consequent increased risk for HC in allogeneic transplantation.