This study aimed to evaluate the effect of prolonging polymerization time on the 18 months clinical performance of a universal adhesive system used as etch-and-rinse (ER) and self-etch (SE). 140 restorations were randomly placed in 35 subjects according to the following groups: ER10 (light-curing for 10 s); ER40 (light-curing for 40 s); SE10 (light-curing for 10 s) and; SE40 (light-curing for 40 s/1200 mW/cm2). A resin composite was placed incrementally. The restorations were evaluated at baseline and after 18 months by using the FDI criteria. The following outcomes were evaluated: marginal staining, marginal adaptation, spontaneous post-operative sensitivity and recurrence of caries. The differences among the groups were calculated using Friedman repeated measures analysis of variance rank (α = 0.05). After 18 months the retention/fracture rates were 88.9% (95%CI 74.1-95.6%) for ER10, 94.3% (95%CI 91.4-98.5%) for ER40, 77.2% (95%CI 60.1-87.9) for SE10 and 97.2% (95%CI 85.5-99.9%) for SE40. A significant difference was detected in the retention rate when SE10 was compared to the SE40 (p = 0.01). Also, ER40 showed a significant improvement of the marginal adaptation when compared to ER10 (p = 0.01). This is the first study that demonstrates an improvement in the clinical performance of universal adhesive systems in non-carious cervical lesions when prolonged light-cured was applied.