We compared the efficiency of immunophenotyping using flow cytometry (FCM) and a combination of morphologic and immunocytochemical studies for detecting malignant cells in 92 effusions. Cytologic results were as follows: carcinoma cells, 43 specimens; benign, 42 specimens; suggestive of nonepithelial malignancy, 7 specimens. After immunocytochemical analysis, 5 benign specimens were reclassified as malignant and 4 malignant epithelial specimens as benign. With FCM, cells positive for Ber-EP4, B 72.3, AH6, and HB-TN were detected in 28 to 36 (64%-82%) of 44 carcinomas but only 2 to 12 (5%-29%) of 41 benign specimens. Significant association was seen for coexpression. Ber-EP4 and AH6 were the most sensitive; Ber-EP4 was the most specific. The presence of cells positive for 3 of 4 markers strongly suggested malignancy (34/44 carcinoma specimens [77%]; 3/41 reactive specimens [7%]). The presence of cells positive for all 4 markers was diagnostic of malignancy (17/44 malignant specimens [39%]; 0/41 reactive effusions [0%]). FCM and immunocytochemical resultsfor Ber-EP4 expression showed excellent association. FCM is a powerful tool for diagnosing difficult effusions and can quantify coexpression of various markers in fresh specimens. By using established cellular markers coupled with biological markers, FCM also has great promise for experimental purposes.