Thoracoscopy with a single opening was carried out under general anesthesia with a cold light thoracoscope of 7 mm in diameter. Biopsies were taken under visual control for optical and electron microscopic examination. In a series of 40 mesotheliomas, 30 patients with pleural effusion underwent 36 thoracoscopies. In 29 patients the macroscopic lesions were compatible with the diagnosis of mesothelioma. The most specific aspects consisted in white-yellowish nodules, almost translucent, arising from the parietal pleura (9 patients). Another typical pattern, a dense pleural thickening, was found in 10 patients. In the remaining patients, the lesions observed were less specific. The biopsy was positive in 35/36 thoracoscopies (97%). A talc poudrage was carried out in 10 patients: their mean survival was 458 days. In ten matched mesothelioma patients who underwent pleurectomy the mean survival was only 395 days. In 12 patients at an early stage, the visceral pleura was not involved by the tumor: their mean survival was 636 days. In 11 patients both parietal and visceral pleura were involved by the tumor: their mean survival was 138 days: (p less than 0,001). It is concluded that in mesothelioma pleural effusions, thoracoscopy is a safe and efficient procedure in order to reach a precise diagnosis, prognosis and efficient palliative treatment of the effusion.