Review of the records of 243 cases of cytologically diagnosed non-Hodgkin's lymphomas (NHL) revealed pleural effusions in 21 (8.6%). Cytologic examination of pleural fluid was done in 17 cases, of which 16 were reported as positive. Cytologic examination was supplemented with cytochemical staining (acid phosphatase, alpha naphthyl acetate esterase and periodic-acid-Schiff reactions) and E-rosetting studies in 12 cases. Of the 16 positive cases, 11 were malignant lymphomas consisting of convoluted lymphocytes. Acute lymphatic leukemia of the prothymocytic type (T-ALL) and chronic lymphocytic leukemia of the T-cell type (T-CLL) comprised one case each, and there were three cases of follicular center cell lymphomas, two of the cleaved-cell type and one of the Burkitt-type. Comparison of the cytomorphology of the tumor cells in the pleural effusion with those in fine needle aspiration smears from the solid tumors in 14 cases showed an identical appearance in 13 cases; in one, the Burkitt-type lymphoma, the cells were larger and more pleomorphic in the pleural effusion. This study indicates that the cytologic diagnosis and categorization of NHL of the convoluted-cell type is greatly enhanced by the study of neoplastic lymphocytes in a pleural effusion.