BACKGROUND Fluoride mouthrinses have been used extensively as a caries-preventive intervention in school-based programmes and individually at home. OBJECTIVE To determine the effectiveness and safety of fluoride mouthrinses in the prevention of dental caries in children and to examine factors potentially modifying their effect. METHODS We searched the Cochrane Oral Health Group's Trials Register (May 2000), the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library Issue 2, 2000), MEDLINE (1966 to January 2000), plus several other databases. We handsearched journals, reference lists of articles and contacted selected authors and manufacturers. METHODS Randomised or quasi-randomised controlled trials with blind outcome assessment, comparing fluoride mouthrinse with placebo or no treatment in children up to 16 years during at least 1 year. The main outcome was caries increment measured by the change in decayed, missing and filled tooth surfaces (D(M)FS). METHODS Inclusion decisions, quality assessment and data extraction were duplicated in a random sample of one third of studies, and consensus achieved by discussion or a third party. Authors were contacted for missing data. The primary measure of effect was the prevented fraction (PF) that is the difference in mean caries increments between the treatment and control groups expressed as a percentage of the mean increment in the control group. Random effects meta-analyses were performed where data could be pooled. Potential sources of heterogeneity were examined in random effects metaregression analyses. RESULTS Thirty-six studies were included. For the 34 that contributed data for meta-analysis (involving 14,600 children) the D(M)FS pooled PF was 26% (95% confidence interval (CI), 23% to 30%; p < 0.0001). Heterogeneity was not substantial, but confirmed statistically (p = 0.008). No significant association between estimates of D(M)FS prevented fractions and baseline caries severity, background exposure to fluorides, rinsing frequency and fluoride concentration was found in metaregression analyses. A funnel plot of the 34 studies indicated no relationship between prevented fraction and study precision. There is little information concerning possible adverse effects or acceptability of treatment in the included trials. CONCLUSIONS This review suggests that the supervised regular use of fluoride mouthrinse at two main strengths and rinsing frequencies is associated with a clear reduction in caries increment in children. In populations with caries increment of 0.25 D(M)FS per year, 16 children will need to use a fluoride mouthrinse (rather than a non-fluoride rinse) to avoid one D(M)FS; in populations with a caries increment of 2.14 D(M)FS per year, 2 children will need to rinse to avoid one D(M)FS. There is a need for complete reporting of side effects and acceptability data in fluoride mouthrinse trials.