We report our experience with 6 cases of severe bleeding of the urinary bladder, due to the cytostaticum cyclophosphamide. In 3 cases a thrombocytopenia was proved. One patient developed a shrunken bladder with bilateral reflux. If the bleeding does not stop after discontinuation of cyclophosphamide, introduction of a catheter, removal of clots and administration of whole fresh blood, then a simple urinary derivation operation is advised.