All patients seen in 1975 to 1984 with benign asbestos pleural effusion (BAPE) were studied. In all, 73 exudates occurred in 60 patients, 40 on the left side and 33 on the right. Relapses occurred on the same side in two patients; 11 had bilateral exudates, three of them concomitantly, in the other patients with a free interval of 1-15 years. The mean latency time from the first exposure to asbestos was 30 years, with a range of 1 to 58 years. The effusions lasted from 1 to 10 months, with a median of 3 months. The most common symptoms were pain, fever, cough, and/or dyspnoea; however, 46% of the episodes were symptomless. The total number of thoracocenteses was 66, with removal of 50 to 2000 ml (mean 460) each time. Fifty-three per cent of the pleural fluids were macroscopically haemorrhagic and 26% eosinophilic. Two findings contribute to a better understanding of the entity: first, even a comparatively slight occupational exposure can be sufficient; secondly, BAPE can occur many years after exposure to asbestos, and not only in the first one or two decades.