The diagnostic accuracy of measurement of carcinoembryonic antigen (CEA) and of analysis of the cytological characteristics of pericardial fluid was assessed in 10 patients with benign pericarditis and 7 patients with malignant pericarditis. The mean CEA levels of pericardial fluid specimens from patients with malignant pericarditis were significantly higher than those from patients with benign pericarditis. However, a high CEA level was not seen in a specimen of pericardial fluid which contained nonepithelial tumor cells. Positive cytological findings were obtained in specimens of pericardial fluid from 85.7% of the patients with malignant pericarditis. Correct diagnosis of malignancy was made by CEA assay alone or by cytological examination alone in 85.7% of the patients examined, while a combination of these methods raised the diagnostic rate to 100%.