Objectives:This study was conducted in order to evaluate the effectiveness of a day-center treatment program to promote development of children with pervasive development disorder (PDD) and pervasive development disorder/mental retardation (PDD/MR). Methods:Twenty five children (14 in the PDD group and 11 in the PDD/MR group) participated in a day-center treatment program. They had been enrolled in the whole program for 2-3 years. Their performance was evaluated according to the Preschool Language Scale (PRES), Social Maturity Scale (SMS), and Korean version of the Childhood Autism Rating Scale (CARS). They were grouped by diagnosis at the beginning of the program and the treatment effect was compared. Results:Children who participated in the day-center treatment program showed a significant increase in their PRES and SMS scores and a decrease in their CARS scores. Conclusions:A day-center treatment program is effective for development of children with PDD and PDD/MR.