We describe a 57-year-old man who presented with diffuse abdominal pain, abdominal enlargement, vomitus, dyspnea and a weight loss of 30 kg within 6 months. These acute symptoms were preceded by an episode of ascites and an acute sigmadiverticulitis 7 months ago. Ultrasonography and computed tomography were suggestive of pseudomyxoma peritonei. However, malignant mesothelioma peritonei was diagnosed by open surgery with biopsy for histological examination. Despite R-2-resection of the tumor and following open hyperthermic intraperitoneal chemotherapy with initial remarkable recovery the patient died 5 months after therapeutical intervention. Malignant peritoneal mesothelioma is an extremely rare tumor with great diagnostic and therapeutic difficulties. We report a case including diagnostical work up and the medical surgical therapy of this disease.