The specificity of cytologic examination in effusions is high but the sensitivity is low. Therefore, various ancillary methods for the detection of malignant cells in effusions have been proposed. The presence of an aneuploid cell population is generally considered diagnostic of malignancy. The purpose of this study is to determine whether the routine use of flow cytometry adds to standard cytologic evaluation in effusions. We did flow cytometric DNA analysis in 76 effusions(28 malignant and 48 benign fluids). All the 48 benign effusions were diploid. There were 12(42.9%) aneuploid and 16(67.1%) diploid malignant effusions. Based on these results flow cytometric DNA analysis had a sensitivity of 42.9% and a specificity of 100%. These results suggest that flow cytometric DNA analysis may be a useful adjunct to conventional cytology, but its principal limitation is us relatively low sensitivity.