The aim of this study is to determine the effectiveness of fluoride varnish (Duraflor, 5% NaF, Pharmascience) combined with caregiver counseling in preventing and reducing ECC in Aboriginal communities, in a 2-year randomized controlled trial. Methods: The trial enrolled 1226 children aged 6 months to 5 years from the Sioux Lookout Zone (SLZ) First Nations reserves and 149 from Thunder Bay (NW Ontario). Twenty First Nations communities were randomized to 2 study groups; families in both groups received oral health counseling and restorative dental care provided by the SLZ Dental Program - one group received no fluoride varnish (FV0) and the other received FV 2-3×/yr. The same subjects were examined for the dmft/s indices by trained and calibrated dental hygienists in 2003, 2004 and 2005. A caregiver questionnaire assessed the impact of ECC on the quality of life of the child and the family. The SLZ Hospital provided data on children's dental general anesthetic (GA) procedures. Comparative cross-sectional oral health data for 416 (2003), 687 (2004) and 544 (2005) 3-5 yr-olds attending Junior Kindergarten in Thunder Bay, not receiving fluoride treatments, were collected by the Thunder Bay District Health Unit's dental hygienists and recorders. Results will assist decision makers in developing programs for addressing the problem of ECC in high-caries-risk populations.