For a better understanding of the prognosis after the onset of a malignancy-associated effusion in patients known or subsequently shown to have cancer, survival time was compared with the findings and the date of the first cytologic diagnosis of an effusion. The number of patients studied was 254; 171 had a pleural and 83 a peritoneal effusion. The average survival time was 25.5 weeks, which was about equal for both sites of effusions. After two years, only 6% of all patients were alive. When the cytologic diagnosis of the effusion was "malignant," only 4% survived after two years; when the cytologic diagnosis was "suspicious for malignancy" and "nonmalignant," these figures were 5% and 7%, respectively. This indicates that a cytologic diagnosis of benign or nonmalignant is not a good indicator of a better prognosis in cancer patients for whom benign causes of the effusion have been excluded. There appeared to be a prognostic relationship between the length of the interval from the initial diagnosis of cancer to the time of examination of the first sample of the effusion: a longer interval was correlated with a better survival. When survival time was viewed in relation to therapy, patients whose pleural effusions were only treated by aspiration were found to have a particularly short average survival (13.9 weeks).