The value of pericardioscopy in pericardial effusion of uncertain origin was evaluated in 20 patients, aged from 18 to 77 years, whose pericardial effusion had been diagnosed by ultrasonography; 2 patients presented with clinical signs of tamponade. The cause of the pericarditis was unknown, but the clinical context suggested a malignant disease in 13 patients, tuberculosis in 5 patients and another cause in 2 patients. The pericardium was explored by means of a direct vision, cold-light endoscope, usually a mediastinoscope, introduced by the retroxiphoidal route under general of local anaesthesia. This method made it possible to study the pericardial fluid, examine the pericardial serous membrane, perform biopsies at a distance from the orifice of entry and cleanse the pericardium thoroughly in cases with blood or pus collection. Apart from 2 cases where the examination could not be completed because of an anterior mediastinal mass and a pericardial symphysis, valuable information could be obtained in purulent pericarditis (n = 1), chronic radiation induced lesions (n = 2), metastases (n = 2), haemopericardium (n = 2), and biopsies could be performed in tumoral or suspicious areas. These guided biopsies revealed a metastasis in 3 cases where the pericardial window was negative. No sign of tuberculosis was found in the 5 cases where the disease was suspected. The final diagnoses were: neoplastic pericarditis in 4 cases, radiation-induced pericarditis in 2 cases, purulent pericarditis in 2 cases, haemopericardium in 3 cases and idiopathic or reactive pericarditis in 9 cases. The post-operative period was uneventful, with no major complication ascribable to the procedure.(ABSTRACT TRUNCATED AT 250 WORDS)