This study evaluated the two-year clinical performance of two minimally invasive cavity preparation techniques, bur and laser, in Class I occlusal resin composite restorations. Twenty-seven patients, each having at least one pair of occlusal caries, were enrolled in this study. For each patient, one of the cavities was prepared with a diamond bur, and the other was prepared with Er, Cr:YSGG laser. The cavities were restored with a nanofilled flowable resin composite, Grandio Flow, using an etch-and-rinse adhesive, Solobond M. A total of 108 restorations were placed in molars by a single operator. The restorations were evaluated according to modified Cvar/Ryge criteria. The evaluations were performed at baseline, 6, 12, 18 and 24 months after initial placement by two calibrated operators. The Chi-square and Fisher's exact test were used for statistical analysis. All the patients were available during all evaluated periods, resulting in a recall rate of 100%. The retention rates of the restorations at 24 months were 98.1% for bur and 100% for the laser-prepared group. After 24 months, 5.6% of the bur-prepared and 7.4% of the laser-prepared restorations were rated Bravo in marginal discoloration (p > 0.05). Bur-prepared (9.3%) and laser-prepared (13%) restorations were rated Bravo in marginal adaptation (p > 0.05). There were no significant differences between the two cavity preparation techniques regarding the evaluated parameters (p > 0.05). Both cavity preparation techniques performed equally, with excellent outcomes after a 24-month period.