The outcomes of a randomized clinical trial of a new behavioral family intervention, Stepping Stones Triple P, for preschoolers with developmental and behavior problems are presented. Forty-eight children with developmental disabilities participated, 27 randomly allocated to an intervention group and 20 to a wait-list control group. Parents completed measures of parenting style and stress, and independent observers assessed parent-child interactions. The intervention was associated with fewer child behavior problems reported by mothers and independent observers, improved maternal and paternal parenting style, and decreased maternal stress. All effects were maintained at 6-month follow-up.