Purpose: This study evaluated the 3-year clinical performance of the polyacid-modified resin composite Dyract AP in restorations for Class I carious lesions. Methods: 108 Class I cavities in 21 subjects were restored with Dyract AP. The lesions, which were diagnosed macroscopically with a probe, involved fissures and had reached the dentin, while lateral spread was limited and localized to the dentin. The average facio-lingual width of the cavities was equal to or less than one-third the intercuspal width. Restorations were evaluated clinically by two experienced, calibrated examiners at baseline and at 1, 2, and 3 years, according to the modified Ryge criteria, (color match, marginal discoloration, wear or loss of anatomical form, caries, marginal adaptation, and surface texture). Results: None of the restorations failed at the end of the first year. At 2 years, two restorations required immediate replacement due to caries lesions adjacent to the margins. At 3 years, six restorations had caries lesions adjacent to their margins, and the cumulative rate of success was 93.4%. Color changes and marginal discolorations were found to be statistically significant (P< 0.001) at 3 years, although none of the restorations needed to be replaced. Significant differences were also observed in the caries rate (P< 0.001) at 3 years. Aside from the eight failed restorations, the degrees of color matching, marginal discoloration, wear or loss of anatomical form, caries, marginal adaptation, and surface texture were clinically acceptable at 3 years post-treatment.