Ninety patients with a history of breast cancer and pericardial effusion detected on echocardiography were identified and divided on a clinical basis into three groups. Group 1 consisted of 20 patients who had progressive metastatic breast cancer and echocardiography performed on a routine basis as a part of a clinical trial involving 38 patients. All 20 had small unexpected effusions, and only one patient developed symptomatic malignant pericardial disease late in her clinical course. Group 2 consisted of 32 patients who were without evidence of metastatic disease at the time of positive echocardiography and the etiology was considered benign in all patients. Six patients required pericardiectomy, five for severe radiation induced pericarditis and one for amyloid. No patient developed proven or suspected malignant pericardial disease. Group 3 comprised 38 patients who had known metastatic disease outside the pericardium at the time of positive echocardiography. Nineteen patients in Group 3 had histologically proven malignant involvement during life or at autopsy, and five more had suspected malignant pericardial disease. Ten patients initially were treated with pericardiectomy and 28 patients were managed with systemic therapy alone (24 patients) or with pericardiocentesis (four patients). Among the 12 patients with malignant effusion treated without surgery, proven local progression of pericardial disease occurred in six, with sudden death in two of those patients. No patient treated initially with surgery suffered progression of her pericardial disease. It was concluded that: small, clinically unsuspected pericardial effusions appear to be relatively common in women with metastatic breast cancer; no patient with clinical pericardial disease confirmed on echocardiography and no evidence of metastatic breast cancer developed malignant pericardial involvement; 50% of patients with known metastatic disease and a clinically apparent pericardial effusion had malignant pericardial disease; and nonsurgical therapy in patients with histologically proven or clinically suspected malignant pericardial effusion was associated with a high incidence of progressive pericardial disease.