Two main metabolites of the cytostatic agent 1,3,2-oxazaphosphorine-3(2-chloroethyl)-2-[(2-chloroethyl)-amino]-tetrahydro-2-oxide (ifosfamidd as two different dechloroethylated derivatives. They both can also be derived from ifosfamide in vitro by means of KMnO4-oxidation. The metabolites are stable in organic solutions and they show no cytotoxic activity in biological test systems. In the urine of a patient treated with ifosfamide, nearly 50% of the applied dose were excreted as dechloroethylated derivatives. In the urine of other patients "carboxyifosfamide" was the main metabolite. It is assumed that the therapeutic effect of ifosfamide depends partially on whether side chain oxidation or ring oxidation is the predominant way of biotransformation.