Primary peritoneal mesotheliomas (PPMs) are rare tumors of adults. At our institution, PPMs are treated with a combination of cytoreductive surgery and intraperitoneal hyperthermic chemotherapy (IPHC) in appropriate patients. We present a summary of cytologic features of PPM in 49 positive (malignant) specimens during a 15-year period at 1 institution. Of the corresponding 49 PPM histologic specimens, 46 were epithelial, 2 sarcomatoid, and 1 multicystic mesothelioma. This includes our experience with washing specimens obtained from patients with PPM following treatment with cytoreductive surgery combined with IPHC. The rarity of PPM makes this neoplasm unfamiliar to most pathologists. However, cytologic features can be diagnostic in a majority of cases. We present a summary of cytologic features that, in our experience, we find to be most useful in making or excluding a diagnosis of PPM. To our knowledge, this is the first large series reporting the cytomorphologic features of PPM in peritoneal effusions, pelvic washing specimens, and infradiaphragmatic fine-needle aspiration biopsy specimens.