Background: The Taiwanese government has been promoting early intervention to children with intellectual disability for years, but data on its effectiveness are limited. Methods: We recruited children who were treated for intellectual disability at a teaching hospital and had two IQ tests from 2001 to 2005 and used the difference between the two tests as the indicator of effectiveness. Results: The participants included 23 boys and 13 girls 56.5 +/- 5.9 months of age at the first test and 73.4 +/- 4.9 months at the second. The IQ increased from 57.0 +/- 8.0 to 65.1 +/- 12.3 (p < 0.001). Multi-variate regressions showed that a low maternal educational level, male gender, and a younger age at the first test were significant independent predictors of the effectiveness. Conclusions: Early intervention can improve the IQ of children with intellectual disability, and the earlier the intervention the better. The effectiveness is demonstrable in boys and more prominent in children whose mothers had a low educational level.