Purpose: To evaluate marginal adaptation of Class I restorations in enamel using contemporary one-bottle all-in-one adhesives, stressed by thermocycling (TC) and mechanical loading (ML). Materials and Methods: Ninety-six extracted human molars were prepared (standard Class I cavities: 3 mm deep, 6 mm wide mesio-distally, and 4 mm wide bucco-lingually). Twelve adhesive systems were used: OptiBond FL (OPT), Clearfil SE Bond (CSE) and Adper Prompt L-Pop (PLP) as controls, compared with nine one-bottle all-in-one adhesives - AdheSe One (AHO), Adper Easy Bond (EB), Bond Force (BF), G-Bond (GB), iBond Self Etch (IB), One Coat 7.0 (OC), OptiBond All-in-one (OPA), Clearfil Tri-S-Bond (TSB), Xeno V (XV). All teeth were restored using Filtek Z250 placed in three (one horizontal, two oblique) increments. Enamel margins were evaluated following 21 days of water storage, after thermocycling (2000 cycles: 5°C to 55°C), and after mechanical loading (150,000 cycles, 50 N). After each step, replicas were produced and quantitative SEM margin analysis was performed (200X) using defined criteria. Results: The median values of % "continuous margin" following TC and ML, respectively, were: OPT(98.6/96.2), CSE(95.4/90.9), BF(81.7/68.1), GB(81.1/65.0), OPA(83.0/68.1), OC(64.1/41.3), TSB(59.3/42.2), EB(57.1/42.6), IB(38.4/27.6), PLP(36.6/21.5), XV(45.0/30.0), AHO(17.7/5.4). Statistical evaluation (Kruskal-Wallis test with Bonferroni adjustment, p 0.05) revealed the following ranking for ML: OPT=CSE>BF=OPA=GB>OC=EB=TSB=XV =IB=PLP>AHO. Conclusion: All one-bottle all-in-one adhesives exhibited statistically significant lower marginal qualities in enamel compared to the etch-and-rinse system OPF and the two-step self-etching system CSE. The results obtained for GB, OPA and BF, however, were better than for the other all-in-one adhesives. Keywords: marginal quality evaluation, Class I, adhesive system effectiveness, thermocycling, mechanical loading