Objective: The aim of this ex-vivo study was to evaluate both the external and the internal penetration ability of a resin infiltrant into natural proximal and macroscopically intact white spot lesions, and to merge this approach with the internal tunnel preparation concept. Method and Materials: 20 premolars and 20 molars with proximal subsurface lesions (ICDAS, code 2) and respective radiographic lesion depths extending into the middle third of dentin (D2 lesions) were selected and divided into two groups. Treatment needs were confirmed using digital imaging fiber-optic transillumination and laser fluorescence. Deproteinization (NaOCl; 2%) followed, and lesions of Group 1 (control; n = 20) were etched (HCl; 15%) and externally infiltrated (Icon). Accordingly, the specimens of Group 2 (n = 20) were treated with the resin infiltrant from external; then, internal Class I tunnels were prepared, lesions were internally infiltrated (Icon), and the occlusal cavities were restored (G-ænial Flo X) after etching (H3PO4 gel; 40%). Teeth were cut perpendicular to the proximal lesion surfaces, and percentage infiltrations were analyzed using confocal laser microscopy and a dedicated image manipulation program (GIMP). Results: Regarding the external infiltration, no differences between both groups were detected (P = .114; Mann-Whitney). Additional internal application of the resin infiltrant significantly increased the percentage amount of enamel lesion infiltration (P .0001; Wilcoxon). Conclusion: External and internal infiltration seem to complement the internal tunnel approach, thus remediating the drawbacks of the latter by occluding and stabilizing the porous areas of the proximal caries lesion, and preserving both the marginal ridge and the proximal contact area. Keywords: minimum intervention dentistry, preventive dentistry, resin infiltrant, resin infiltration, subsurface lesion, tunnel restoration