Urine of twenty hospital workers was monitored for the excretion of the cytostatic drug cyclophosphamide using GC-MSD. The drug was found to be present above the detection limit of 0.5 microgram/24 h urine in five cases. A clear relationship between cyclophosphamide handling and the detectability of excretion existed. This method developed can be of use for biological monitoring studies directed toward the finding of exposure hazards.