Studies on the caries-poor oral hygiene relationship were inconclusive. Objectives: To determine whether oral hygiene would be a risk indicator of enamel and/or dentin caries. Methods: a cross-sectional study conducted by clinical dental examination and self-administered questionaire to the parent of 542 seventh grade school children in Rayong, Thailand. The cavitated and non-cavitated criteria were used to score for caries and the Hygiene Analysis Index for plaque. Results: Mean DMFS for enamel and dentin caries of these children were 3.82 and 1.99, respectively. Logistic regression analyses showed that oral hygiene or plaque factor associated only with enamel caries (p=0.047) while maternal education and use of fluoride toothpaste associate with dentin caries (p=0.017, 0.004) Conclusions: This suggests that by the chronic nature of caries progression and dynamic state of oral environment oral hygiene seem to be a risk indicator of only enamel caries. Further cohort study was called for the proposition that oral hygiene may be a good predictor. This study was supported by the Mahidol University Research Grant.