The purpose of this in-vitro study was to compare the caries preventive effects of a fluoride releasing sealant (FS) with a conventional sealant (CS) using an artificial secondary caries system. Twelve primary teeth with caries-free buccal and lingual surfaces were selected for this study. The teeth were sectioned into quarters. Following a fluoride-free prophylaxis, cavity preparations were placed in the buccal and lingual surfaces of the tooth quarters, acid-etched and sealed with FS and CS per the manufacturer's instructions as follows: 1) mesiobuccal and mesiolingual quarters (FS, Fissurit-F, VocoChemie): and 2) distobuccal and distolingual quarters (CS, Fissurit, VocoChemie). The specimens were thermocycled (500 cycles) in artificial saliva and then exposed to an artificial caries system. Longitudinal sections were taken and examined by polarized light microscopy to determine mean surface lesion depth and presence of wall lesions, following lesion initiation (6 weeks of gel exposure) and lesion progression (9 weeks of gel exposure). Mean surface lesion depth was 124 mu for FS and 176 mu for CS (p < 0.05, paired t-test) after lesion initiation, and 167 mu for FS and 238 mu for CS (p < 0.05, paired t-test) after lesion progression. Wall lesions were less frequent (p > 0.05, paired t-test) with FS (21 percent initiation, 29 percent progression) than with CS (25 percent initiation, 38 percent progression). Improved caries resistance in enamel adjacent to a fluoride-releasing pit and fissure sealant may prove to be beneficial in reducing primary surface lesion depth during caries formation and progression, while decreasing caries susceptibility of enamel sealed with a fluoride-releasing pit and fissure sealant.