Clinical and cytologic findings in 21 breast cancer patients with symptomatic pericardial effusion are presented. The etiology of the pericardial effusion was definitely malignant, by cytology/histology in 13 patients (62%), and suspected malignant by cytology in 2 patients (9%). One patient (5%) with definitely nonmalignant pericardial effusion by cytology was found to be histologically positive at autopsy. In 5 patients (24%) there was no histological/cytological evidence of malignancy; radiation pericarditis could be the etiology in 4 of these 5 patients. The median time from the diagnosis of breast cancer to the development of symptomatic pericardial effusion was 60 months (range: 1-219 months). Ten patients developed cardiac tamponade; they were treated by either pericardiocentesis or pericardiectomy. The mean survival of patients with negative cytology/histology was 12 months; patients with suspicious cytology had a mean survival of 9 months; patients with malignant effusion, treated by pericardiectomy, had a mean survival of 22.3 months, while patients with malignant pericardial effusion, who were not subjected to surgery, had a mean survival of 4.7 months, only. It is concluded that the etiology of symptomatic pericardial effusion in breast cancer patients is not always malignant, which emphasizes the role of fluid cytology in establishing definite diagnosis. The survival probability is a function of the extent of extracardiac disease; among patients with malignant pericardial effusion those selected for pericardiectomy have a longer than average survival.