Thirty-four patients receiving allogeneic bone marrow transplants as treatment for haematological malignancy were prospectively randomized to receive or not to receive bladder irrigation by indwelling urinary catheter during preparation for transplant. Twenty-two patients received busulphan and cyclophosphamide, four received busulphan, cyclophosphamide and irradiation, and eight received cyclophosphamide and total body irradiation. The actuarial incidence of haemorrhagic cystitis in those randomized to receive bladder irrigation was 48% for the whole group and 52% in those receiving busulphan and cyclophosphamide only. In those randomized not to receive bladder irrigation the incidence of haemorrhagic cystitis was 29% for the overall group and 38% in those receiving busulphan and cyclophosphamide. There was no statistically significant difference between the two groups. We conclude that bladder irrigation does not minimize the risk of haemorrhagic cystitis in this patient population.