Thirteen cases of hernia sacs exhibiting mesothelial hyperplasia of such a degree as to simulate a malignant process are presented. There were 2 infants, 7 children, and 4 adults. The hernias were inguinal in 12 patients and epigastric in the remaining. In 7 cases (52%) there was a history of trauma or some other source of injury to the sac, such as incarceration. Microscopically, solid nodules of atypical mesothelial cells were seen in the hernia sac, either free or attached to the wall. Mitoses, multinucleated cells, and "strap cells" were sometimes present. Clues to the mesothelial nature of the proliferation were the cytologic features and the presence of similar changes in the lining of the hernia sac.