This phase I study was carried out to determine the maximal tolerated dose of carboplatin (Car) together with a fixed dose of etoposide (E) and to recommend the optimal dose for a phase II study. The dose of E was 100 mg/m2 given i.v. on days 1-3, and the starting dose of Car was 200 mg/m2 given i.v. on day 1. The dose was escalated until WHO grade 4 toxicity developed after two treatment cycles in more than one-third of the patients. A total of 33 patients with advanced lung cancer entered the trial. The maximal tolerated toxicity of the combination was reached at a dose of 500 mg/m2 Car. Myelosuppression was moderate, and hematological toxicity of WHO grade 4 was encountered in one of five patients at 475 mg/m2 and in two out of five patients at 500 mg/m2. The main toxic effects were leucopenia and thrombocytopenia. The frequency of treatment-related infections was low and no deaths were caused by treatment. There was a significant overall correlation between the platelet nadir and creatinine clearance. One complete response and three partial responses were achieved after two treatment cycles. Based on the results of the present study, the dose of carboplatin (combined with 100 mg/m2 eposide given on days 1-3) recommended for phase II studies is 450 mg/m2.