The mutagenic and cancerogenic antineoplastic agent cyclophosphamide (CP) is released into sewage water by cancer patient excretion. To assess the biological degradability of CP two standardized test systems, the Zahn- Wellens/EMPA test (OECD 302B) and a laboratory scale sewage treatment plant, were used. In both test systems the agent exhibited only poor degradability. To verify the expected occurrence of CP in hospital sewage, water samples were analyzed for CP with GC/MS after enrichment by solid-phase extraction. CP could be detected in concentrations ranging from 20 ng/L to 4.5 μg/L. The occurrence of the agent could also be proved in samples from the influent and the effluent of the communal sewage treatment plant into which the hospital's sewage water is shed. Concentrations ranged from 7 to 143 ng/L. In an attempt to assess the contribution of CP to the genotoxicity detected in hospital waste water in a recent study, the effects of CP in the umuC test, a bacterial genotoxicity assay, were investigated. However, no genotoxic effects of CP were found up to concentrations of 1 g/L.