The purpose of this laboratory study was to evaluate caries-like lesion formation in occlusal enamel adjacent to a light-cured resin-modified glass ionomer utilized as a pit and fissure sealant and a conventional light-cured, fluoride-releasing sealant. Fluoride-free prophylaxis was done on occlusal surfaces of 12 caries-free mandibular molar teeth that had not been exposed to the oral cavity. Occlusal surface morphology was examined by SEM on the uncoated specimens. Each tooth was then sectioned into 2 portions buccolingually, producing mesial and distal tooth halve. Occlusal surfaces of mesial tooth halves were prepared for an experimental light-cured resin-modified glass ionomer (RMG) sealant (PH-SE II, ESPE), and for comparison, a light-cured fluoride-releasing pit and fissure (PFS) sealant (Helioseal F, Ivoclar) was placed on occlusal surfaces of the corresponding distal tooth halves. The sealed occlusal surfaces were examined uncoated by SEM to compare RMG and PFS adaptation. After thermocycling in artificial saliva, caries-like lesions were formed in the occlusal surfaces adjacent to RMG and PFS. Longitudinal sections were taken for comparison of lesion formation adjacent to RMG and PFS. Mean lesion depths in occlusal surfaces were 64 +/- 17 mm for RMG, and 116 +/- 27 mm for PFS (p < 0.05, paired t-test). Occlusal lesions terminated at the point where bonding occurred between the occlusal enamel and RMG or PFS. SEM surface topography demonstrated adequate adaptation of the materials with obliteration of the typical pit and fissure surface morphology by both RMG and PFS. While both the resin-modified glass ionomer and fluoride-releasing sealant materials protected the pit and fissure enamel from caries development, the resin-modified glass ionomer reduced the extent of caries involvement in the adjacent unsealed occlusal incline enamel, when compared with the resin sealant.