100 cases of institutionalized adult mental retardation are divided into three groups according to the presumed etiology. 1. Familial group (so called familial mental retardation); that is, some pathological or organic change in the brain are excluded from the clinical symptom and past history, and same kind of mental retardation is found in some of their family members. 2. Idiopathic group; clinical symptom, past and family history suggest no precise etiology. 3. Pathological group; 19 cases of so called familial mental retardation are presented and compaired with the other two groups. Social and mental age are very low and abnormal findings on EEG or PEG are found in most of these cases. From these results, complicated relation between intelligence and pathology of brain are suggested.