The malignant mesothelioma is a rare tumour with a rising incidence during the past decades [Lancet 1 (1986) 1275; Br J Int Med 28 (1971) 59; Am J Int Med 9 (1986) 397]. The tumour is derived from the mesothelial lining of the pleural cavity, the pericardium or the peritoneum. Mesotheliomas solely involving the peritoneum are particularly rare. Two cases of malignant peritoneal mesotheliomas are described to demonstrate the difficulties in diagnosing this disease. Surgical exploration, either by laparotomy, or preferably by laparoscopy, has been proven valuable as a diagnostic method.