The ThinPrep® Processor is gaining popularity in both gynecologic and nongynecologic cytologic samples, including effusion specimens. We compared immunocytochemical results on ThinPrep and cell-block preparations from the same effusion specimen with antibodies commonly used in effusion cytopathology. Samples from 17 reactive effusions and 79 effusions with metastatic adenocarcinomas were each prepared as monolayer ThinPreps and formalin-fixed, paraffin-embedded cell blocks. All slides were immunostained with antibodies against intermediate filaments (cytokeratins 8 and 18, and vimentin), cytoplasmic or membrane proteins (EMA, polyclonal CEA, B72.3, and BerEP4), and nuclear antigens (Ki67, PCNA, and p53), using the avidin-biotin procedure. ThinPrep and cell-block slides were stained simultaneously, using identical protocols. Both preparations showed similar results with respect to frequency of positivity, intensity, and distribution of stain for all nonnuclear markers tested. However, for the three nuclear markers Ki67, PCNA, and p53, the frequency and intensity of reaction with ThinPrep were significantly lower than with the cell-block preparation, particularly in malignant effusions. These findings suggest that immunocytochemical results obtained with ThinPrep® match those of cell block for most markers tested. However, cell-block preparation are superior to ThinPrep for nuclear markers (Ki67, PCNA, and p53). Diagn. Cytopathol. 2003;28:1–5. © 2003 Wiley-Liss, Inc.