Two cases of diffuse malignant mesothelioma of abdominal cavity were analysed. These tumors arise from the peritoneum and are also found in the parietal and visceral pleura, pericardium and in vaginal tunic. All of them, infra or supra-diaphragmatic, are associated with asbestos exposure in at least 80% of cases. It is difficult to explain how inhaled asbestos induces peritoneal neoplasms. This aspects become very important in the diagnostic, basically why it is done at laparotomy or laparoscopy. When was proceed the biopsy of the lesions, and occasionally by identification of malignant mesothelial cells in ascitic fluid. In this two cases exposed considerations about the advanced phase of diagnostic are made, the diagnostic was performed in the majority of the collected cells, showing the advanced stage of the disease. At that time of diagnosis we observed poor evolution. We call attention to the importance of precancer diagnosis, the best chance to treatment options, always based on surgical resections, radiation or chemotherapy alone or combined. If the radical surgery is not possible, this patients must be treated by chemotherapy or radiotherapy, defined after complete staging of the disease.