This study explores the level of service effectiveness perceived by the service users of early intervention services for the children with ADHD in Daegu. Also, this study tries to find the key factors influencing the level of service effectiveness. Based on other studies on the service effectiveness, this study categorizes the related factors into four groups: child factors, family factors, service use factors, and service quality factors. The study sample was drawn from the service users living in Daegu, based on the lists of almost all the service organizations providing this service. Data were gathered through the questionnaire disseminated to 363 service users, and analyzed using descriptive statistics and multiple regression analyses. The study findings show that the level of service effectiveness perceived by service users is relatively high. The most important factors influencing on the level of service effectiveness are the level of service quality. The level of service quality is positively associated with that of the effectiveness perceived by service users. Among the different dimensions of service qualities, the quality of assurance and responsibility are the most powerful factors predicting the level of service effectiveness. Also the variables indicating whether or not service users are welfare recipients, the amount of money users pay, the types of service organization users utilize are significantly influencing on the level of the service effectiveness.