The aim of this study was to evaluate interdental caries incidence and progression in relation to the effect of antibacterial varnish treatments in schoolchildren anticipated at caries risk. After a screening procedure, 110 healthy children (8-10 years) with moderate and high counts of salivary mutans streptococci (MS) were invited to join a 2-year longitudinal study. At baseline, MS were enumerated at all mesial interdental sites of the first permanent molars with a chair-side technique. The children were then treated 3 times within 2 weeks by interdental spot applications with a 1% chlorhexidine/thymol-containing varnish (Cervitec). Follow-up samples of saliva and plaque were collected 1, 3, 6, and 12 months after onset of treat. Caries prevalence, incidence and progression of the selected approximal surfaces were scored from bitewing radiographs exposed at baseline and after 2 years. Sixty-three children of the same age formed a non-varnish-treated reference group. Reduction of caries incidence and progression was dearly dependent on the outcome of the antibacterial treatment. A significantly (P< 0.01) higher progression score was found among children who exhibited a less marked suppression of interdental MS levels compared with those with high suppression and the children of the reference group. The results suggest that a suppression of MS in interdental plaque may be an important event to prevent and arrest approximal caries development in schoolchildren at risk. Monitoring the effect of antibacterial agents in a site-specific way could therefore be advocated.