Three familial cases of idiopathic hypertrophy of the masticatory muscles are reported. The muscular enlargement was noted in late childhood or early adult life, was bilateral, insidious, painless and non-progressive. Electrophysiological and laboratory findings were normal in one case. CT examination, performed in two cases, showed homogeneous bilateral enlargement of masseter muscle bulk consistent with true hypertrophy. Two patients also showed congenital malformations. A review of the literature showed that this disorder is very rare.