A comparison was made of the efficacy of pleural needle biopsy and pleural-fluid cytopathology in the diagnosis of pleural tumor in a group of 271 patients. A malignant tumor involving the pleura was present in 95 cases. Needle biopsy alone provided a diagnosis of tumor in 53 instances, and cytopathologic preparations were diagnostic in 69 patients. A diagnosis was established on either the biopsy or cytopathology, or both, in 86 cases (90 percent). These results indicate the value of using both biopsy and fluid cytology in the evaluation of pleural effusion, which often is due to involvement of the pleura by malignant neoplasm.