A number of commercially prepared chlorhexidine mouthrinses which are now available are formulated at concentrations lower than the more usual 0.2%. This study compared 0.12% and 0.1% chlorhexidine mouthrinses for effects on plaque regrowth and gingivitis, using a two 19-day period single-operator blind-crossover study design on 14 healthy human volunteers. The 0.12% rinse was a commercial product previously shown as effective as a 0.2% rinse. The 0.1% rinse was a reformulated version of a 0.1% preparation commercially available at the time of this study. Plaque reformation was recorded on days 12 and 19 by score and area. Gingivitis was recorded at day 1, 12 and 19 by measuring gingival crevicular fluid, gingival index and bleeding on probing. The mouthrinses were used twice a day and as recommended by the manufacturer. Mean scores for plaque and gingivitis were mostly lower with the 0.12% rinse but only reached significance for plaque score on days 12 and 19 and for plaque area on day 19. Reformulation of the 0.1% would appear to have markedly improved the antiplaque properties to levels similar to a known effective commercially available 0.12% rinse.