AbstractScanning Electron Microscopic (SEM) examination of body fluid specimens submitted for cytopathological diagnosis is technically easy and can be accomplished within hours. Specimens refrigerated for as long as 72 hr show no significant structural alterations that would interfere with recognition of diagnostically significant cells. SEM is particularly well suited for these specimens because the cells are free floating, and even cells in clusters have “natural” surfaces. Clinical uses of SEM on body fluids include the identification of ambiguous cells, distinction of reactive (benign) mesothelial cells from metastatic adenocarcinoma, and distinction of lymphoid cells from small cell carcinoma of the lung. In addition, SEM of these fluids assists cytologists to better understand the cellular features and associations seen in the light microscope. Ultrastructural analysis can be an important component of the Quality Assurance Plan for those cytodiagnostic laboratories that have access to an electron microscopy facility. © 1992 Wiley‐Liss, inc.