X-linked alpha-thalassemia/mental retardation syndrome (ATR-X), which was first reported by Wilkie, et al. in 1991, is a disorder with severe mental retardation, characteristic facial appearance, genital abnormalities, and mild form of alpha-thalassemia. At present, about 50 cases have been reported in the world, but few in Japan. We report 3 cases of this disorder in 2 families. All cases prefer a peculiar posture and show unique movements, such as self-induced vomiting or self-hanging, which can be diagnostic. ATR-X should be considered as a differential diagnosis in all male patients with severe mental retardation.