Etiology and pathogenesis of MCA/MR in 1,023 patients (618 male; 405 female) with mental retardation were studied. Of 1,023 patients, there were 563 cases (317 male; 246 female) with MCA (55%). Among the MCA patients, there were 303 (156 male; 147 female) whose primary etiology was clarified (53.8%). Among the 260 patients with MCA/MR of unknown etiology, there were 23 with recognizable syndromes of unknown etiology and 7 previously reported by us as possibly having a new malformation syndrome. We had 569 patients with mental retardation of unknown etiology including 263 (41.5%) who were involved with MCA.