OBJECTIVE The objective of this study was to evaluate the effect of chlorhexidine-thymol varnish on the prevention of caries lesions in primary molars among schoolchildren ages 6 to 7 in relation to their previous experience with caries. METHODS Two groups of schoolchildren of lower-middle socioeconomic level were followed up in a clinical trial: one group of 86 children, treated with a chlorhexidine-thymol varnish (Cervitec) and another group of 95 children who served as controls. The varnish was reapplied every 3 months, and the caries lesion increments were compared at 24 months. RESULTS There was no statistically significant difference between these 2 groups in the increment in decayed and filled primary molars. The children in the varnish group with no decayed or filled primary teeth at baseline showed a significantly lower (P<.05) incidence of caries lesions in primary molars (at 24 months) compared with the control group. CONCLUSIONS Chlorhexidine-thymol varnish can be said to reduce caries lesions in the primary molars of schoolchildren ages 6 to 7 with no previous caries lesion experience.