In many cases of chronic microembolic pulmonary hypertension the source of the emboli is unknown. Disease or injury to the sinus node is usually accompanied by thrombus formation within the antrum atrii dextri, and this region is not often inspected carefully during necropsy. In the present study the hearts of 14 patients with right ventricular hypertrophy or documented pulmonary hypertension showed disease or injury in the sinus node, and each had both old and recent thrombosis in the antrum atrii dextri. In all 14 cases both lungs were available for histological examination. The group included six cases of rheumatic heart disease with mitral stenosis, four cases of disseminated lupus erythematosus, and four cases of so-called primary pulmonary hypertension. Every lung contained both new and old microemboli, as well as a wide variety of other histological abnormalities narrowing the small pulmonary arteries. Many of these narrowing lesions were recognisable as the consequence of prior microembolisation. For future postmortem examination of cases of chronic microembolic pulmonary hypertension, mural thrombi within the antrum atrii dextri should be considered as one important potential source for recurring microembolisation to the lungs.