This article evaluates the effectiveness of a developmentally based early intervention programme. Two groups of children were compared, a treatment group and a no-treatment control group. Standardized assessments were administered before and after the intervention period by an independent clinician. Pre-treatment comparisons revealed that the control group had a significantly higher pre-treatment IQ; but the two groups were comparable for age, mental age, socioeconomic status and number of hours of non-experimental therapy. Results demonstrated that children in the treatment group improved significantly more than those in the control group on measures of joint attention, social interaction, imitation, daily living skills, motor skills and an adaptive behaviour composite. A measure of requesting behaviour fell short of statistical significance. The total stress index reduced for treatment group parents and increased for the control group parents (but not significantly). The results of the study are considered to support the efficacy of this treatment approach.