This study reports the cytopathologic diagnoses rendered on all malignant pleural effusions received and processed over a period of 14 years. Specimens of fluid from various body sites (25,464) were examined. Of these, 5888 (23%) were specimens of pleural effusions. Five hundred eighty-four specimens (9.9% of total pleural fluid specimens) taken from 472 patients were diagnosed as containing cancer cells. Of the malignant pleural effusions, 75.7% were classified as carcinomatous in type. Adenocarcinomas comprised 47.4% of the 584 specimens. The groups of large cell undifferentiated carcinoma and lymphoma/leukemia approximated one another in being the second most common cancer groups (14.3% and 15.0%, respectively). For both males and females, the frequency of organ site or primary tumor type was lung (35.6%), lymphoma/leukemia (15.9%), breast (14.8%), female genital tract (8.1%), and gastrointestinal tract (5.9%). Among male patients, the order of frequency was lung (49.1%), lymphoma/leukemia (21.1%), gastrointestinal tract (7.0%), genitourinary tract (6.0%), and malignant melanoma (1.4%). In female patients, the order of frequency was breast (37.4%), female genital tract (usually ovary) (20.3%), lung (15.0%), lymphoma/leukemia (8.0%), and gastrointestinal tract (4.3%). In 48 patients (10.2%) the primary site of neoplasm was never determined. In 90.5% of patients a cytopathologic diagnosis conclusive for cancer was obtained on the first specimen of fluid. There were no false positive diagnoses.