To examine the frequency and diagnostic implications of malignant pericardial effusions.The clinical and histologic records of pericardial effusions diagnosed cytologically as malignant were reviewed. We investigated the relationship between malignancy and type of tumor, interval between diagnosis of the primary tumor and development of the pericardial effusion, and length of survival after the onset of the effusion.There were 375 pericardial effusions among 23,592 effusions studied over 24 years; 65 of them were diagnosed as malignant. The mean age at onset was 53.6 years. In 92% of the cases the primary tumor was epithelial, lung the most frequent in males and breast in females. In 48% of cases the pericardial effusion constituted the first sign of malignancy.Diagnosis of a pericardial effusion as malignant carried an ominous prognosis: 86% of patients died within the first year and nearly one-third within the first month. Breast carcinomas were linked with the longest survival and longest interval of latency prior to the onset of the pericardial effusion. There were two effusions associated with lymphoblastic lymphomas; they have had no recurrence 10 and 17 years after the effusion, and the patients can be considered cured.