The European Lung Cancer Working Party has investigated prognostic factors for response, overall survival, long term survival in a population with advanced non small cell lung cancer registered in a clinical trial evaluating platinum derivatives-containing chemotherapy regimens. Various factors have been identified in multivariate analyses and were classified using RECPAM methodology. In addition to the clinical factors such as disease extent and performance status were shown, as significant predictors, rarely studied biological factors like pretherapeutic leucocyte and polynuclear levels. The obtention of an objective response to chemotherapy appeared to be a major prognostic factor for further survival.