Among 25 persons with monosymptomatic exudative pleurisy of unknown etiology there were 11 men who had been exposed to asbestos. Ten of them were investigated during an observation period of 4 to 8 years. The exudate was more or less sanguinolent, had a prolonged course and recurred on the same or opposite side in the majority of cases. Large numbers of eosinophilic cells were present in seven exudates. Clinical symptoms were very discrete. After the disappearance of the exudate, persisting pleural thickening was present in nine patients. Decortication was performed in four of these patients and signs of non-specific chronic pleurisy was found. Asbestos bodies were found in one case and suspected in another. No other etiological signs were observed in any of the 11 men during the period of observation. At the end of the period of observation, radiological signs of mild basal pulmonary fibrosis were found in one patient and signs suggestive of this were present in a further three patients. Six of the men were troubled by dyspnoea. Three of these had reduced physical working capacity owing to respiratory factors. It is concluded that asbestos should be suspected as the causative factor in monosymptomatic exudative pleurisy if other etiological factors can be excluded.