The aim was to evaluate the effect of Eco-CIMT in young children with unilateral cerebral palsy in a randomized controlled crossover design. The training was implemented within the regular pediatric services, provided by the child's parents and/or preschool teacher and supervised by the child's regular therapist. Methods: Twenty-five children (mean age 28.8 months [SD 11.2], 72% male) participated. Assisting Hand Assessment (AHA) was used as the outcome measure. The Eco-CIMT was provided for 2 h a day over a period of two months. Children were randomized into two groups and started either with Eco-CIMT or as controls with a four-month washout period before crossing over. Result: A significant effect of Eco-CIMT was found when compared to the control period, and the estimated treatment effect was 5.47 (95% C.I. 2.93-8.02) (including both Group 1 and Group 2) (p less than 0.001). The non-significant estimated carryover effect allowed us to collapse the two groups based on estimates from the ANOVA model. No clear relationship to hours of training, age or general attitudes of mastery was found. Conclusion: Eco-CIMT influenced development more than ordinary treatment at this age when Eco-CIMT was performed by parents and preschool teachers supervised by the child's ordinary therapist. (Contains 3 figures and 3 tables.)