The annual incidence of clinically manifest pericarditis was found to be 0.34% in 157 females and 0.44% in 77 males with rheumatoid arthritis, observed for a mean time of 5.7 years. The development of pericarditis was independent of disease duration, but was related to extensive joint involvement, subcutaneous nodules, and a high Waaler Rose titre. Concomitant pleural effusion was present in four of five patients. Pericarditis in rheumatoid arthritis may indicate a serious prognosis quo ad vitam. Four of the five patients died within eight years, but their age at death was relatively high (62-73 years).