The authors report a case of mesothelial cyst of the diaphragm in a boy 11 years old who was examined for pain in the right hypochondrium with exacerbation during respiratory movements. Ultrasonography and CT suggested the diagnosis. However the final diagnosis of mesothelial cyst of the diaphragm was possible only after laparotomy and histological examination. The topographical, clinical, radiological, therapeutic and histological aspects of primary cysts of the diaphragm are presented and a survey of the literature is made.