Different types of peritoneal mesothelioma (PM) occur in children and adults. All these share certain histopathologic features but differ in other aspects, such as age of occurrence, site and sex predominance, etiology, and biologic behavior. The article describes four patients, two with cystic PM (one of whom had multiple recurrences) and two with malignant PM (one of whom had pleural metastases). These cases illustrate the variable behavior of this tumor in childhood and highlight the difficulties encountered in diagnosis and treatment. Three different groups of mesothelioma are recognized: a classic, asbestos-related, malignant mesothelioma of adults, typically occurring in the pleural cavity; a multicystic mesothelioma, predominantly affecting the pelvic peritoneum of young women and associated with a good prognosis; and mesotheliomas in children, which are not associated with asbestos exposure and have an unpredictable biologic behavior requiring individual treatment strategies. In the patients studied, DNA index measured by flow cytometry showed a difference between the cystic (aneuploid) and malignant (diploid) tumors. The proliferative rate (S phase) of the tumor was low in all four cases.