The "Intensive Early Intervention Program for Children with Autism" (IEIP) is a program funded by the province of Ontario. It is used to teach/treat young children who have been formally identified as having an autistic spectrum disorder. Intensive Behavioural Intervention (IBI) services are provided to these children, aged 2 to 5 years, who meet specific program requirements. The program was designed taking into consideration the central tenets of Applied Behavioural Analysis (ABA), which is a widely recognized and accepted method for teaching functional skills to children with autism. In this paper, we review the effectiveness of Intensive Behavioural Intervention for teaching/treating young children with autism. The effects of age, duration of therapy, and number of hours of therapy are examined in an effort to determine whether or not there would be an increase in the participants' IQ, adaptive functioning, and language abilities after receiving intensive services from the program. With reference to this, data on three children with autism are presented in an attempt to isolate and more thoroughly examine outcomes. Overall, the data suggests that the program does indeed work for some young children with autism, however, unfortunately, not for all. More specifically, two of the children demonstrated some gain in the areas evaluated, including IQ, adaptive functioning, and language ability, whereas, the third child did not appear to make any significant progress in any of the formally assessed areas. Interestingly, the participants' age, the duration of therapy, and the number of hours of therapy did not appear to conclusively influence overall treatment outcome. In a review of the literature, and, as is demonstrated by our own study, Intensive Behavioural Intervention has been used with varying degrees of success to treat young children with autism, however, it appears that the effectiveness of this treatment is dependent on factors that perhaps have not yet been discovered, or yet fully explored.