Malignant mesotheliomas of the hernial sac are uncommon and only a few cases have been diagnosed incidentally during herniorrhaphy procedures. The prognosis is poor and patient management is difficult because current treatment modalities do little to prolong survival. Molecular markers could be useful to identify potential therapeutic targets. Using microarray-comparative genomic hybridization (aCGH), two cases of peritoneal mesothelioma that were found incidentally at the time of hernia repair, were investigated. A high number of genetic aberrations was detected in both cases. The gains were prevalent. The tumors showed identical lost regions at 2q13, 6q25.3, 6q26, 6q26→q27, 9q31.1→9q31.3, 10p15.3, 11q13.2, 13q14.2, 19q13.42→q43, and gains at 1p36.33, 3q29, 5p15.33, 7p22.3, 10p15.1→10p14, 11q13.2, 12q24.23, 12q24.33, 16p13.3, 17p13.3, 18p11.31, 19q13.43, 21q21.1→q21.2, 22q11.1→q11.22, Xp21.2, Xq28. Survival was longer in the patient with a lower total number of genetic defects. aCGH provides a high-resolution map of copy number changes that may be critical to mesothelioma progression.