The families of Down's Syndrome children and another group with a similar degree of retardation were compared using data collected at interview. Differences in social class distribution were explained by the maternal age effect in the Down's group. Divorce was more common in the control group but the quality of marriage in those parents still together and the health of parents were similar. No close associations were found between behaviour disorders in the children and family factors, except that behaviour disorders were likely to be associated with similar disturbance in siblings next in age and to be more common in the less happy marriages.