To evaluate the role of compomers in secondary caries formation in vitro.36 human teeth with caries-free enamel (coronal) and root surfaces underwent a fluoride-free prophylaxis. Class V cavities were prepared in buccal and lingual coronal and root surfaces. The teeth were divided into two groups with 3 treatment subdivisions [12 teeth per restorative material (6 with coronal restorations and 6 with root surface restorations)]). The preparations were restored with: (1) filled composite (Filtek Z250 with Single Bond), (2) fluoride-releasing compomer [Hytac with Prompt L-Pop (fluoride-releasing water-based adhesive)]; or (3) fluoride-releasing compomer [Dyract AP with Prime & Bond NT (fluoride-releasing acetone-based adhesive)]. 24 cavities (6 teeth with 2 cavities/tooth) were available with each treatment. The teeth were thermocycled (500 cycles at 5-50 degrees C) in synthetic saliva prior to artificial caries formation. After lesion formation, 5 longitudinal sections were taken from each specimen for polarized light evaluation. Mean surface lesion depths and wall lesion frequencies were determined.The Filtek Z250 group had significantly (P 0.05, ANOVA, DMR).