Late pericardial effusion (PE) after open heart surgery has a potential for serious complications, including tamponade necessitating urgent drainage. From October 1990 to March 1995 moderate to massive PE developed following 9 of 359 (2.5%) cardiac procedures between 11 and 55 days postoperatively (mean, 18.6 days). Only one of these nine patients had evidence of cardiac tamponade; the other 8 patients had moderate symptoms including malaise, weight gain, and dyspnea on exertion. All patients except one were being treated with warfarin and anti-platelet agents. Echocardiography detected posterior PE of variable magnitude in all 9 patients; anterior PE was present in only one of these patients. Pericardiocentesis under echocardiographic guidance using an echo transducer with a built in puncture needle was performed in all patients. A flexible catheter was left in place for 24 to 72 hours for suction. In each case, 300 to 712 ml of old bloody fluid (mean, 475 ml) was evacuated, with relief of symptoms. Despite continuing anticoagulant therapy, there was no recurrence of pericardial effusion. The technique we have described simplifies pericardiocentesis and helps to avoid complications in this procedure.