This study examined the psychological adjustment of children with diabetes and their siblings. Data were collected for 41 diabetic children and their siblings between ages 6 and 11 yr who were followed in a large pediatric practice. The diabetic group was matched with 35 control children who were followed for routine pediatric care and their siblings. Diabetic families and the controls did not differ on divorce rate or level of marital adjustment. Behavior problems were measured through the use of the Achenbach Behavior Checklist for both sexes separately at the 6-11- and 12-16-yr-old age levels. Diabetic boys, aged 6-11 yr, were higher than nondiabetic boys on internalizing and externalizing symptom scales, particularly on measures of schizoid tendencies, obsessive compulsive symptoms, hyperactivity, and aggression. Neither female patients, male siblings, nor female siblings in the 6-11-yr-old age group differed from controls on emotional adjustment. Among the 12-16-yr-old males, diabetic patients were higher on internalizing symptoms than male siblings and their controls. Male siblings tended to be lower than diabetic patients and controls on school performance. Social class differences may have contributed to the differences at the 12-16-yr age levels. Female subjects in the 12-16-yr-old group did not differ on measures of adjustment. Level of control was unrelated to patients' or siblings' adjustment, but behavioral symptoms tended to increase for male diabetic patients with length of illness, and tended to decrease for male siblings. Results are interpreted according to social class effects and sex differences.