Approximately half of 351 body-cavity effusions from 263 patients were examined prospectively in paraffin-embedded cell blocks and in smears, while the other half were examined in smears alone. The number of suspect and positive fluids obtained with the combined cell block-and-smear technic was double that of specimens examined in smears only. No false-positive case was found. Tumors were subsequently demonstrated in 38% of the patients who had negative or atypical cytologic reports. Smears stained with the Papanicolaou technic generally have good definition of malignant cellular changes, wheras cell blocks are particularly useful when the cytologic abnormalities are misleading, such as in reactive mesothelial cells, or obscure, as in occasional well-differentiated adenocarcinomas. It is recommended that both cell blocks and smears be used in evaluating all fluids submitted to the cytology laboratory.