A retrospective analysis of 5,255 pleural effusion specimens from 3,811 patients was undertaken to determine the accuracy of cytopathologic correlation with pleural biopsy, the detection rate of malignancy by cytology, and the frequency distribution of malignant effusions according to age group. The cytopathologic correlation was 96.5% accurate, with 0.1% false-positive results and 0.18% false-negative results by cytology. The sensitivity of cytologic detections was 6.7% higher than that of pleural biopsy. Frequency analysis showed that the incidence of carcinoma of the lung, the most common cause of malignant effusion, is not sex based. Adenocarcinoma of the lung was the most frequent type of malignancy found in pleural effusions. It represented 79% of lung carcinomas that metastasized to pleura, accounting for 40% of all malignant pleural effusions. In young adults, lymphoreticular malignancies were the most common cause of malignant effusions.