Abstract The aim of this study was to investigate whether the use of a pre-brushing mouthwash (Plax) improved subjects' oral hygiene over a 2-week period. In a double-blind placebo controlled cross-over study, 33 adult subjects (mean age 35.3 years) used either Plax or a placebo mouthwash for 30 s prior to tooth brushing during a 2-week period. After a washout period of 4 weeks, the subjects used the other mouthwash in the same way. The modified plaque index of Quigley and Hein (QHI) was recorded for 16 teeth (16, 14, 13, 11, 21, 23, 24, 26, 36, 34, 33, 31, 41, 43, 44, 46) at each visit. The bleeding index (BI) was recorded at the beginning and end of the study. Results were analysed using a paired one-tailed t-test. Neither plax nor placebo mouth washes reduced plaque scores significantly (p>0.05) below baseline. There was no statistically significant difference between the mean BI at the beginning of the trial (0.42) and at the end (0.44). These results suggest that the routine use of Plax is not a useful adjunct to toothbrushing.