A total of 289 cases with malignant pleuroperitoneal effusions were reviewed, and their prognosis were analyzed in relation to the following factors: (1) chronologically divided series; (2) previous tumorectomy of the primary lesion; (3) presence or absence of other metastases; (4) site of effusion; (5) site of the primary lesion; (6) cytologic characteristics of effusion. Prognostic difference among each chronologically divided series was not noticed. Previous tumorectomy slightly improved the patient's survival. Other distant metastases had no significant influence on the prognosis. Pleural effusion gave better prognosis than peritoneal effusion. Patients of breast and lung cancer had longer expectancies, contrasting with those of gastric cancer. Formation of large clusters of cancer cells on smear preparations obtained from the tapped effusions was interpreted as a sign of good prognosis. Frequencies of cancer cell and lymphocyte occurrence on smear preparation did not have any relationship with the prognosis. The results showed that some factors of the patients at malignant effusion accumulating stage did contribute a great deal to their prognosis.