A technique for pericardioscopy at the time of subxiphoid pericardial window was evaluated in 17 patients undergoing surgery for clinically suspected malignant pericardial effusion. Best results were obtained using a rigid mediastinoscope for inspection of the posterior and lateral pericardial surfaces. No complications ensued, although many patients experienced cardiac arrythmias which always resolved with removal of the scope. Pericardioscopy revealed cancer transgressing the pericardium near the pulmonary veins in one patient, and this would have been missed without pericardioscopy. Pericardioscopy confirmed palpable metastatic deposits on the inferior pericardial surface in two other patients. In 14 patients, pericardioscopy did not reveal malignancy, although four of these patients had both positive fluid cytologic findings and malignant infiltration of the pericardial biopsy. In one patient a palpable but not visible nodule was proved to be an extrinsic hepatic metastasis. Pericardioscopy is a safe intervention chiefly applicable in patients with central tumors and pericardial effusion in whom subxiphoid pericardial window is not clearly diagnostic at the time of surgery.