In his excellent book Malignant Effusions. A multimodal Approach to Cytologic Diagnosis(Igaku-Shoin, Tokyo, 1996) - chapter 4 - Serosal reaction to Injury, p.26-51, and in chapter 8, Mesothelioma, Dr. Carlos W M Bedrossian discusses at length the great variability of mesothelial benign and malignant cells. This is important to know, to help to distinguish malignant mesothelial cells from their benign reactive or hyperplastic variants, or from metastatic adenocarcinoma cells. In this case we see several morphological characteristics of those malignant changes: doublets, triplets, or mosaics with windows between the cells ( caused by microvilli),clasplike connections, multinucleation, anisonucleosis, cytoplasm with sharp ecto-endoplasmic demarcation, extreme chromatin irregularity, nuclear infoldings,etc. Surgical biopsy confirmed the diagnosis, with calretinin immunohistochemistry positive reaction.