Signet-ring adenocarcinoma cells in serous fluids have been classically described as possessing vacuolated cytoplasm and eccentrically placed, crescent-shaped nuclei. We studied serous fluids from six patients that contained signet-ring adenocarcinoma cells by light microscopy; one case was also studied by transmission electron microscopy. We found that the adenocarcinoma cells were more often present in a non-signet-ring configuration. The typical crescent-shaped nucleus was rarely displayed in smears and may be seen only in the cell-block preparation. Special stains (PAS, mucicarmine and Diff-Quick) showed globular cytoplasmic positivity in signet-ring adenocarcinoma cells but not in mesothelial cells. Significant characteristic electron microscopic findings in the signet-ring adenocarcinoma cells included (1) cytoplasmic lumens or invaginations or both, (2) cytoplasmic protrusions and (3) mucin granules of various sizes and densities. Singly or in combination, all of the above features were located on one side of the nucleus, which suggests that signet-ring adenocarcinoma cells retain some degree of cellular polarity.