Thirty-one patients with squamous cell carcinomas of the oral cavity who underwent intra-arterial induction chemotherapy with 5-fluorouracil and cisplatin were analysed by DNA flow cytometry. Seven tumors consisted exclusively of diploid cells and 24 were characterized by the presence of an aneuploid tumor cell line. Response to chemotherapy as well as survival were significantly better in the diploid group. The cytotoxic effect of the treatment on aneuploid tumor cell populations was monitored in vivo by serial DNA flow cytometric measurements. Decreasing numbers of aneuploid cells were detected in all but two cases (92%); twelve (50%) showed a complete disappearance of the aberrant cell line at the end of chemotherapy. All non-responders showed persisting aneuploid tumor cells at the end of treatment. Even if response to chemotherapy was better in the group with a complete disappearance of the aneuploid cell line in the course of treatment, the survival outcome of these patients did not differ significantly from those who showed persistence of aneuploid tumor cells.