AIM: Cystic peritoneal mesothelioma is commonly regarded as a benign neoplasm at its first manifestation. It can only seldom show malignant transformation, and only after repeated postoperative recurrences. MATERIAL OF STUDY: We hereby represent a unique case of peritoneal cystic mesothelioma, malignant since its first presentation. We observed a 73 year-old man presenting with intermittent abdominal pain and periumbilical swelling. At surgery, we found an extensive, oval-shaped, multi-lobed cystic formation that was surgically removed. RESULTS: Histopathology was consistent with a malignant peritoneal cystic mesothelioma. In agreement with oncologists, we decided not to give any further therapy because of the few possibilities offered by systemic chemotherapy and the paucity of published data from the literature. We planned periodical follow-up including US scan every six and CTscan every 12 months. DISCUSSION: Several cases of malignant transformation occurring after repeated recurrences of peritoneal mesothelioma have been reported. To our knowledge, this is the first case showing “ab initio” histological features of malignancy, typical of an active, proliferating and infiltrating lesion. In addition, advanced age and male gender of our patient are extremely peculiar CONCLUSION: Our observation suggests the possibility, although very rare, that peritoneal cystic mesothelioma may present as malignant since its first manifestation.