A 6-yr double-blind study was undertaken to compare the caries inhibiting-effectiveness of (a) 1 mgF- tablets (daily at school) plus fortnightly rinsing at school with 1000 ppmF-; (b) 1 mgF- tablets plus placebo rinsing; and (c) placebo tablets plus 1000 ppmF- rinsing. Participants were aged 4.5-5 yr at outset of the trial. At baseline, and annually thereafter, clinical caries and bitewing X-ray examinations were undertaken, hence ethical approval for a negative control was not feasible. Fissure sealant presence was also recorded. Initially, 192 children from predominantly low socio-economic backgrounds were enrolled, and baseline analyses showed no significant differences between groups with respect to primary caries prevalence. After 6 yr, 112 children remained, by which time no significant differences were noted between DMFT and DMFS values for those in the active tablet/active rinse group, as compared to those in the placebo tablet/active rinse group. However, in relation to both indices, in permanent first molars, the effectiveness of the active tablet/placebo rinse regime appeared to be significantly poorer than the placebo tablet/active rinse programme. For the active tablet/active rinse group, the DMFT difference was significantly less than the active tablet/placebo rinse group (37.9%), although for the DMFS difference, significance was not achieved. Similar trends were noted when DMFT and DMFS values for all permanent teeth were considered. The occlusal caries prevalence in permanent first molars followed the DMFT pattern, but differences between combined tablet/rinsing children and tablet-only children could be ascribed solely to the effectiveness of sealant presence.(ABSTRACT TRUNCATED AT 250 WORDS)