OBJECTIVE: This study evaluated the importance of enamel at the cervical margin for support and retention of a class II composite restoration in relation to fracture strength, fracture mode, and leakage. METHODS: Sixty-five newly extracted teeth were randomly divided into five groups. Within each group, standardized class II preparations were made at the mesial surface of the tooth with four different preparation designs. Group D (n=15) had the cervical margin placed below the cemento-enamel junction (the dentin group), and in the other three groups (the enamel groups: E1, E2, and E3), the cervical margin was within the enamel (n=15 each). Group E3 had restorations with cuspal coverage, while groups E1 and E2 differed in vertical dimension. Intact teeth without preparation or restoration were tested as controls (n=5). The area of the horizontal part of enamel at the cervical margin of the preparation (available cervical enamel) was calculated. The teeth were restored with a nanofilled composite material and an etch-and-rinse adhesive system. The teeth were subjected to artificial aging consisting of thermocycling and mechanical cyclical loading. The restorations were subsequently loaded until fracture. The teeth were examined microscopically to assess fracture mode and leakage at the interface between the restoration and the tooth substance. RESULTS: The fracture strength in group D (without cervical enamel) and E3 (with cuspal coverage and cervical enamel) was lower (p<0.01) than in the other two groups (with cervical enamel). There was a correlation between the area of available cervical enamel and fracture strength (p<0.01). The median fracture strength for the control teeth was not significantly different from groups E1 and E2. Group D exhibited a predominance of adhesive fractures, while the other groups revealed more cohesive fractures. CONCLUSION: The results from this study indicate that available cervical enamel has an impact on the performance of class II composite restorations.