A phase II study of cis-dichlorodiammineplatinum (II) (CDDP) was performed in 83 patients with inoperable non-small cell lung cancer (21 squamous cell carcinomas, 57 adenocarcinomas and 5 large cell carcinomas). CDDP was given by i.v. infusion at a dosage of 50 mg/m2 (low-dose regimen) or 80-100 mg/m2 (high-dose regimen) every 3 to 4 weeks. Seventy-nine out of 83 patients were evaluable for tumor response. Sixteen of these evaluable patients achieved complete or partial response, and the overall response rate was 20.3%. The response rates were 27.8% in patients with squamous cell carcinoma and 19.6% in those with adenocarcinoma. None of the patients with large cell carcinoma responded. The high-dose regimen was superior to the low-dose regimen in response rate (25.6% versus 5.8%). The responders survived significantly longer than patients with NC (p = 0.02) or PD (p = 0.003). Leukopenia of less than 3000/mm3 occurred in 12.1% of cases, and thrombocytopenia of less than 7 X 10(4)/mm3 occurred in 7.6%. A transient elevation of serum creatinine value was observed in 9 patients (10.8%). Moderate to severe nausea and/or vomiting occurred in almost all patients. It was therefore considered that CDDP was one of the most effective agents for non-small cell lung cancer with tolerable toxic effects.