This study evaluated the effect of sealants on enamel demineralization, focusing on physical protection of the sealed enamel and fluoride protection of the adjacent unsealed enamel. Occlusal fissures with areas measuring 12 mm2 were delimited in 48 extracted molars, randomly divided into 4 groups (n =12): 1) no sealing; 2) sealing with a resin-modified glass-ionomer (Vitremer, 3M ESPE); 3) sealing with a fluoride-releasing composite sealant (Clinpro Sealant, 3M ESPE); and 4) sealing with a non-fluoridated composite sealant (Concise, 3M ESPE). A 4-mm2 window was outlined on the buccal enamel for analysis of fluoride uptake. Following treatment, groups 2, 3 and 4 were subjected to 5-days of pH-cycling, while group 1 was kept in a moist environment at 37 degrees C. Fluoride uptake was assessed by dental biopsy, and the amount of fluoride released to the cycling solutions was determined by ion analysis. Enamel demineralization around the sealants was evaluated by cross-sectional micro-hardness analysis. Group 2 showed higher levels of fluoride release (P < 0.01) and uptake by enamel (P < 0.05), and lower levels of demineralization (P < 0.05) than groups 3 and 4. Group 3 exhibited reduced demineralization on unsealed enamel and provided fluoride uptake in a distant enamel area, while group 4 did not.