To distinguish reactive mesothelial cells from malignant cells in body fluid, we applied silver staining of nucleolar organizer regions(AgNORs) to ethanol fixed cytologic preparations. Fifty aspirated samples of benign(22 cases) and malignant(26 cases) body fluids were studied using the one step silver staining method. Two cytologically atypical samples were also included in the study. In malignant cases the mean AgNOR count was $3.56{pm}0.81$, while in benign cases the mean AgNOR count was $2.02{pm}0.33$. The difference of AgNOR counts between these two groups were statistically significant(p<0.001). The mean of atypical cases was 2.91. Both were diagnosed as malignant in follow-up cytology. In malignant effusions, there is statistically significant difference in AgNOR counts between cells forming complex papillae or clusters and singly scattered cells(p<0.05), $3.29{pm}0.95;and;3.83{pm}0.55$, respectively. We concluded that AgNOR count appears to be useful as a diagnostic tool especially when the cytologic differentiation is difficult.