The purpose of this study is to investigate the effectiveness of a group-based family intervention program known as the Group Positive Parenting Program (Triple P), with families in Japan. Reductions in children's behavioral problems, changes in dysfunctional parenting practices, and affects on parenting adjustment were examined. Participants of both the intervention and control groups (N = 91 and N = 24, respectively) were recruited from mothers visiting health clinics in Kawasaki City, Kanagawa. Intervention and control groups were assessed in terms of child behavior (Strength and Difficulty Questionnaire, SDQ), parenting style (Parenting Scale, PS), and parenting adjustment (Depression-Anxiety-Stress Scale, DASS; and Parenting Experience Survey, PES), both pre- and postintervention. A repeated measures multivariate analysis of variance was used to determine the intervention's effects. The SDQ score for the conduct problems subscale indicated a significant intervention effect. In addition, the postintervention scores for all subscales of the PS, the DASS depression subscale and total scores, as well as ratings for perceived difficulty of parenting in the PES, were significantly reduced in the intervention group alone. The PES also revealed that confidence in parenting significantly increased only in the intervention group. Group Triple P is effective in decreasing child conduct problems, dysfunctional parenting practices, depression, anxiety, stress, and the perceived level of parenting difficulty, as well as in improving parenting confidence, among Japanese families. (Contains 2 tables.)