The aim of this study was to evaluate the morphology of the enamel surface along margins of class V restorations following exposure to cariogenic solution. Restorations were placed in vitro in human third molars. The specimens were divided into groups according to resin composition: (1) Scotchbond 1 + Filtek Flow, (2) Scotchbond 1 + F2000, and (3) Prompt L-Pop + experimental flowable composite. Samples were stored in a demineralizing solution (lactic acid, pH 4.5, 0.1 M) at 37 degrees for 1-4 weeks or in deionized water (control group). The solution was changed every day. Replicas of the specimens were obtained in order to exclude drying artifacts. Scanning electron microscopy (SEM) of original and replica specimens identified a distinct enamel zone, defined as perimarginal enamel showing numerous fractures, porosities, voids, and pits. After the 4-week treatment, perimarginal prismatic enamel was greatly removed, while interprismatic enamel was still in place and only partially dissolved. Enamel not in relation with composite/compomer margins (0.5-1 mm away) showed minor alterations. Perimarginal enamel fractures probably due to composite/compomer shrinkage or the bur preparation may greatly contribute to this marginal enamel demineralization by increasing the number and size of porosities, that enhance the penetration and diffusion of cariogenic solution and create a sort of demineralized enamel subsurface. Only compomer restorations revealed a thin caries inhibition zone (1-2 micro m) probably related to fluoride release. Below this protected area, we observed the typical alterations of the other samples. These morphological alterations are probably related to secondary demineralization lesions and may affect the clinical life of restorations.