Cis-Dichlorodiammine platinum (II) (cis-DDP) was demonstrated to be a potentiator of radiation therapy (RT) in experimental tumor models and in cultured cells. To assess the effectiveness of a combined modality treatment including RT and a weekly low-dose administration of cis-DDP, from January 1986 to June 1987, 95 patients with unresectable locally advanced non-small cell carcinoma of the lung (stage IIIa, b) were randomized for study. Fifty patients received RT alone at doses of 50 Gy; 45 patients received the same RT plus cis-DDP 15 mg/m2 IV weekly. An overall response rate of 50% and 64% was observed in the RT and RT + cis-DDP group, respectively. No statistically significant differences were detected with regard to median survival time (11 months for RT v 16 months for RT + cis-DDP) and progression-free interval (7 months in the RT arm v 9 months in the RT + cis-DDP arm), but the patterns of the first failure appeared to be affected by treatment. In fact, a lower number of intrathoracic relapses was observed in the RT + cis-DDP arm (12 in the RT + cis-DDP v 23 in the RT arm). Toxicity was mild and the feasibility of this schedule must be remarked. A better local control of disease can be obtained using cis-DDP as a radiation potentiator, but the true influence of this combined modality treatment on the length of survival, and the optimal cis-DDP timing and dosage are still to be evaluated in further clinical trials.