Background: Liquid-based cytology (LBC) testing induces morphologic changes due to the use of specific fixatives and preparation techniques, and the cytologies of effusions determined in this manner differ morphologically from those of conventional cytopreparation (CCP) smear methods. We compared the cytologic features of pulmonary small cell carcinoma in effusion fluid using CCP and LBC preparations.Methods: Fifty-three malignant effusion specimens from 36 patients with small cell carcinoma were examined, including 41 LBCs from 27 patients and 12 CCPs from 9 patients.Results: LBC and CCP preparations preserved the typical features of small cell carcinoma, that is, nuclear molding, very high nuclear to cytoplasmic ratio and granular chromatin. The architectural patterns involved small cohesive clusters and chains with nuclear molding, tight three-dimensional clusters, or single cell dispersion were preserved in both preparations. Oval nuclei (83.3% vs 26.8%, P < .001) and a discernable rim of cytoplasm (66.7% vs 26.8%, P = .043) were more frequently identified in CCPs, whereas cellular degeneration and dry artifact were more frequent in LBC preparations (73.2% vs 8.3%, P < .001). LBC had a tendency to show frequent nuclear size variation (51.2% vs 25.0%) than CCP.Conclusion: LBC tends to show more degeneration and dry artifact with exaggerated irregular nuclear shape and nuclear size variation and scanty cytoplasm than CCP.Cytopathologists should be familiar with the cytomorphologic spectrum of this tumor in CCP and LBC prepared effusions. K E Y W O R D Sconventional cytology, effusion, liquid-based cytology, lung cancer, small cell carcinoma