Abstract： Objective To compare the penetration abilities of resin infiltration into proximal lesions in primary molars with those of adhesive in vitro.Methods Thirty-two extracted or exfoliated primary molars showing proximal white spot lesions were selected.Roots of the teeth were removed,and the crowns were cut across the white spot lesions perpendicular to the surface.Cut surfaces were examined (by stereo microscopy) and classified with respect to histological lesion extension (C1-C4):lesions confined to the outer half on enamel(C1),lesions confined to the inner half on enamel(C2),lesions confined to the outer half on dentin(C3),lesions extending into the inner half of dentin (C4).Corresponding lesion halves were etched for 120 s with 15％ hydrochloric acid gel and were subsequently treated with either adhesive or resin infiltration.Specimens were observed with laser scanning confocal microscope(LSCM) in dual fluorescence mode.In confocal microscopic images,lesion depth and penetration depth of the resin infiltration or the adhesive in corresponding halves were measured,and penetration percentages were calculated respectively.Differences of the data between two groups were analyzed by Wilcoxon signed rank test.Variations of histological caries extensions were detected with Kruskal-Wallis H test.Results At the same grading level (C1-C3) in histological caries extension,penetration depths of the resin infiltration group and the adhesive group were 240(230,260) μm vs 190(150,210) μm,405(300,523) μm vs 180(160,200) μm,and 590(430,640) μm vs 180(160,200) μm respectively.There was significant statistical difference in the data between two groups (P＜0.05).Statistically significant difference in penetration depths of the resin infiltration group (at C1-C3) were found (P ＜ 0.01).At the same grading level (C1-C3) in histological caries extension,percentage penetrations of the resin infiltration group and the adhesive group were [100.0％ (96.2％,100.0％)],[99.1％ (95.7％,100.0％)],[82.0％ (81.1％,92.2％)] and [79.2％ (68.4％,87.5％)],[41.8％ (29.1％,74.5％)],[30.2％ (29.2％,39.6％)],respectively.The difference between the above data was also significant(P ＜ 0.05).Percentage penetrations of the resin infiltration group at C1 and C2 level was higher than those at C3 level(P ＜ 0.05).Conclusions The resin infiltration is capable of penetrating almost completely into proximal lesions in primary molars.