We describe four patients with bilateral pleural effusions progressing to diffuse pleural thickening for which we have been unable to find any evidence of an infective, embolic or occupational aetiology. In order to avoid confusion with diffuse pleural thickening attributable to asbestos-related disease, the term cryptogenic bilateral fibrosing pleuritis is suggested. The patients differed from those with pleural shadowing due to asbestos in that none of them gave a history of asbestos exposure, all were ill, presented with chest pain which was not always pleuritic in character, and had dyspnoea, cough or malaise. They had pleural effusions of variable size, pleural shadowing radiographically and raised sedimentation rates. Computed tomography revealed bilateral extensive pleural thickening in all cases. All four were HLA B44 positive. Histology showed that in all cases the pleura was thickened by fibrous tissue. Both layers were affected and the pleural space was often obliterated. Otherwise the pleural surface was covered by organizing fibrin. Focal collections of lymphocytes were often present when the fibrous tissue abutted on the subpleural fat. No asbestos bodies were seen in any of the cases and in one patient electron microscopic fibre counts showed no excess of asbestos. Pleural decortication was successful in three patients. In one of these, contralateral disease was successfully controlled with corticosteroids, but the fourth patient has not improved on corticosteroids.