The aim of the present study was to evaluate the effect of fluoride on the progression of erosive demineralisation in human enamel and dentine using a cyclic de- and remineralisation model in vitro. The mineral content expressed in micrometres was determined daily by longitudinal microradiography (LMR) and presented as cumulative mineral loss over 5 days. For erosive demineralisation, all samples were immersed in 0.05 M citric acid (pH 2.3) for 6x10 min/day and stored in a remineralisation solution. Fluoridation measures were performed as follows: group 1: control, no fluoridation; group 2: toothpaste fluoridation 3x5 min/daily (NaF, 0.15% F-); group 3: toothpaste fluoridation as group 2 and additionally application of a fluoride mouthrinse (Olaflur/SnF2, 0.025%F-) 3x5 min/daily and on days 1 and 3 gel fluoridation (Olaflur/NaF; 1.25% F-) for 1x5 min. After the first experimental day, no significant differences were found between the groups. However, after 5 days the erosive mineral loss values for enamel were 147.5+/-18.7 microm in the control group, 128.1+/-15.0 microm in group 2 (p< or =0.05) and 116.1+/-12.4 microm in group 3 (p< or =0.001). In dentine, the respective values were 136.7+/-16.4, 111.8+/-26.9 (p< or =0.001) and 60.3+/-17.8 (p< or =0.001). The intensive fluoridation significantly reduced erosion progression in enamel but had a more pronounced effect on dentine. The results suggest that subjects with erosive lesions should use an intensive fluoridation measure.