The initial metabolism of ifosfamide (IFO) consists of two different pathways, which lead to the alkylating metabolite 4-hydroxy-IFO and to chloroacetaldehyde (CAA). CAA has been reported to cause side effects, such as neuro- and nephrotoxicity, whereas no direct antitumor effect has been described thus far. Therefore, two human tumor cell lines (MX1 and S117) and a renal tubular cell line (Landa Leiden) were exposed to 4-hydroxy-IFO, CAA, and a combination of both. The concentrations used were in the same range as measured in the blood of 10 patients treated with 5 g/m2 IFO. The cell survival was measured using the MTT assay. Similar dose-response curves were found for both metabolites. For the MX1 tumor, the IC50s of 4-hydroxy-IFO and CAA were 10.8 and 8.6 µm, respectively. For the reduction of S117 cell survival, higher concentrations of the metabolites were needed (25.0 µm 4-hydroxy-IFO and 15.3 µm CAA). Combination treatment of the cells resulted in an approximately additive effect. Both metabolites exhibited similar toxicity against Landa Leiden cells. Our results indicate that CAA has its own cytotoxic profile against tumor cells. Hence, we conclude that the molecular mechanism of action of IFO seems to be only in part an alkylating effect and that CAA may play an important role in the therapeutic efficacy of IFO.