A total of 71 cases of pleural effusion in patients with and without cancer were analyzed by conventional cytology and flow cytometry (FCM) in order to detect cells with an abnormal DNA content (aneuploidy). For cytologic examination, the samples were prepared using standard techniques. Sample for FCM analysis were centrifuged and exposed to hypotonic solution containing detergent and propidium iodide. Thirty-eight patients had pleural effusion due to benign disease, whilst 33 patients had primary lung cancer. All 38 patients with benign pleural effusions showed FCM diploidy. There were 17 aneuploidy (52%) and 16 diploidy (48%) in the 33 patients with lung cancer by FCM analysis. Four of these 33 effusions were cytologically negative, however, FCM showed aneuploidy in 2 of these 4 patients. Based on these results, FCM analysis combined with conventional cytopathology yielded 100% specificity, 94% sensitivity and 100% predictive value of positive result. There were no false-positive results but 2 false-negative results. These findings suggest that FCM is a rapid and useful technique in the analysis of pleural effusion and can be a very useful adjunct to conventional cytopathology.