Résumé : Purpose: To identify pretreatment variables predicting response to platinum derivatives containing chemotherapy in patients with unresectable non-small cell lung cancer (NSCLC). Patients and methods: Eligible patients included in one of the 7 consecutive clinical trials conducted by the European Lung Cancer Working Party between December 1980 and August 1991. All patients received a cisplatin or carboplatin containing chemotherapy. We analyzed 22 potential prognostic factors including sex, age, histology, performance status, weight loss, type of lesions, extent of disease, main metastatic sites and several biological parameters, namely white blood cell count (WBC), neutrophil count, platelet count, hemoglobinemia, creatininemia, serum alkaline phosphatases and LDH. Results: On 1052 eligible patients, 107 were not assessable for response. The objective response rate was 26% (95% C.I.: 23-29%). Univariate analysis identified as statistically significantly associated with a higher objective antitumoral response rate the following characteristics: a normal platelet count, the absence of skin metastasis, the absence of adrenal metastasis, a higher creatininemia, a normal hemoglobinemia, an older age and a normal WBC count. On a restricted set of variables including data from 777 patients, a multivariate logistic regression model disclosed age and platelet count as significantly and independently related to response rate. Conclusion: Clinical and demographic characteristics of patients with unresectable NSCLC, as well as routine laboratory parameters, could not accurately predict response to chemotherapy in a population of patients selected for a clinical trial. Future studies on this subject should include more sophisticated variables as new biomolecular markers.