During a cross sectional medical survey of 2815 insulation workers with 30 years or more from onset of asbestos exposure conducted from 1981 to 1983, a positive history of benign pleural effusion was found in 20 (0.71%). Two or three such episodes had occurred in four of these 20 subjects. The chest x ray abnormalities in these cases were characterised by pleural fibrosis in 19 and diffuse pleural fibrosis with blunting of the corresponding costophrenic angle in 16. In the total group of 2815 insulation workers diffuse pleural fibrosis was found in 142 (5.0%). Thus diffuse pleural fibrosis with blunting of the corresponding costophrenic angle is a frequent residual abnormality after benign pleural effusion. Its impact on pulmonary function can be pronounced.