AbstractNucleolar organizer regions are loops of DNA associated with silver‐stainable proteins (AgNORs). In general, malignant cells have more and larger AgNORs than benign cells. An inconsistent argyrophilic method and difficulties a objectively evaluating AgNORs account for some of the reluctance to utilize AgNOR staining as a diagnostic tool to differentiate benign and malignant lesions. Section from paraffin‐embedded cell blocks of 10 cases of benign and nalignatnt peritoneal effusions were stained with a modified AgNOR method. Backscattered electron imaging in the Scanning electron microscope, together with image analysis, was used to evaluate more objectively a number of AgNOR parameters and to determine which measurement was the most reliable discriminant of the two types of fluids. One hundred nuclei per case were identified and imaged. In contrast to benign nuclei, AgNORs in malignant nuclei were more numerous (P0.0001). and larger (P0.0001) A cut‐off mean AgNOR area of 1.1 μm2(P0.0001) correctly categorized all malignant (1.1 μm2) and benign (>1.1 μm2) cases. This system's objectivity and specificity could be and to enhance the cytological interpretation of effusions, where the separation of reactive mesothelial cells and malignant cells is extremely d