We performed an investigation focusing on the distribution of tumor types responsible for positive pleural effusions in 143 patients who died of malignancy and underwent autopsy. The principal malignant tumors were lung carcinoma (41 cases, 51.2%) and pleural mesothelioma (23 cases, 28.7%) in males and breast carcinoma (24 cases, 38.2%) and lung carcinoma (13 cases, 20.6%) in females. Histologically, most of the cases belonged to the adenocarcinoma category. The first morphologic diagnosis was a cytologic one in 86 cases (60.1%), especially regarding lung cancer. In breast cancer a positive pleural effusion always preceded recurrent disease with a rapidly progressive course, even a long time after the initial surgery. The results of this study, based on both cytomorphologic features and postmortem data on the tumor sites, may be a useful working framework for the cytologist dealing with a positive pleural effusion.