In 157 women and 77 men with rheumatoid arthritis, observed for a mean of 5.7 years, the annual incidence of pleural effusion was found to be 0.34% in the women and 1.54% in the men. In 4 pleural fluids the fluid-to-serum ratio for Waaler-Rose titres was 1:4, which agrees with the ratios for other proteins and does not suggest a pathogenetic role of the rheumatoid factor. In two other pleural fluids the findings suggested immune mechanisms implicating the rheumatoid factor. In general, the data, both clinical and any other, was compatible with the possibility of multiple etiological and pathogenetic factors in rheumatoid pleural effusions. These were: extrinsic factors causing pleural thickenings in 25% of non-rheumatoid men and in less than 10% of non-rheumatoid women aged more than 50 years; preceding lung disorder; particular inflammatory responsiveness due to rheumatoid disease, related or unrelated to rheumatoid factor; and some degree of transudation.