Familial patterns of mental retardation were examined among white and black children in the NINCDS Collaborative Perinatal Project population. Among whites, the mildly retarded children had more affected relatives than did the severely retarded, consistent with the traditional two-group theory of mental retardation. In blacks, where differences in family patterns between the mildly and the severely retarded were less clear, most retardation appeared to be of the cultural-familial type.