A continuous series of 125 patients hospitalized between 1973 and 1987 with pleural malignant mesothelioma were the subjects of this stepwise analysis. A diagnostic thoracoscopy was carried out in every patient and the histopathological diagnosis confirmed by the French Mesothelioma Panel of Pathologists. The main characteristics of the patients were: mean age 64 +/- 1 year, 104 male, 21 female; previous asbestos exposure was present in 98. The histopathological type was epithelial in 90, mixed in 22, fibrosarcomatous in 13. The endoscopic aspect was analysed in the same manner from the beginning as follows: involvement of the parietal, diaphragmatic or visceral pleura; extension of the tumor into the pleural cavity, macroscopic aspects of the lesions. According to the Butchart's classification 56 patients were at stage I, 61 at stage II, 3 at stage III, 5 at stage IV. A multivariate analysis (Cox model) showed that the best prognostic factor is the stage I which can be divided in 2 subtypes: exclusive involvement of the parietal and diaphragmatic pleura without invasion of the visceral pleura: stage IA (median survival 31.2 months). It is 6.75 months as soon as the visceral pleura is invaded (stade IB).