In general, treatment of childhood obesity focuses on treating the obese children. The results of child-directed treatments are disappointing in the long run. In the current study, it is tested whether a treatment aimed solely at obese children's parents results in positive effects on the children's weight status. In addition, potential predictors of treatment success are identified. The parents of 98 overweight or obese children (aged 7-13 years) were randomly assigned to either the cognitive-behavioural group treatment (eight sessions) or the waiting-list control group. With respect to child body mass index (BMI) percentile, the parents' treatment was successful in reducing overweight from pretreatment to posttreatment: BMI percentile decreased significantly by 2.4% in the treatment group, whereas there was no change in the waiting-list control group. There was no significant relapse at follow-up (3 months). Child BMI percentile did not decrease in the waiting-list control group. In addition, significant main effects of time were found for both groups with respect to eating psychopathology (decrease), self-esteem (increase) and negative thoughts (decrease). Finally, parental BMI decreased significantly only in the treatment group. Four predictors were identified with respect to treatment success, namely, lower socioeconomic status, younger age of the child, higher parental attendance and lower BMI percentile of the child before treatment. The parents' treatment had significant effects on child and parent BMI. Long-term endurance of these positive effects needs to be studied. Striking are the positive effects of time in the waiting-list control group for some psychological outcome measures. Obviously, waiting for treatment already affects psychological processes (but not behaviour) in the children.