Objective: This study evaluated the clinical efficacy of 2 flowable resin composites used to restore occlusal carious lesions. Tetric Flow (Vivadent) and Esthet-X Flow (Dentsply/Caulk) are resin composites with decreased filler loading and lower viscosity compared to conventional resin composites. Method and materials: Sixty occlusal restorations (30 of each material) were placed. After tooth preparation and caries removal, each material was inserted with rubber dam isolation. Prime & Bond NT (Dentsply/Caulk) was used to bond both flowable composites, which were incrementally placed and light-cured for 20 seconds. Each restoration was evaluated at baseline (1 week after restoration placement), 3 months, 6 months, and 1 year for marginal discoloration, secondary caries, anatomic form, retention, polishability, marginal adaptation, and color match. Groups were compared at 1 year and the data analyzed statistically with a .05 level of significance. Results: A marginally significant difference in color match was observed at 1 year (favoring Esthet-X Flow). No other differences were observed between materials. Marginal discoloration significantly worsened for all restorations at 6 months and 1 year, and marginal adaptation significantly worsened at 1 year for all restorations. A positive association between preparation size and sensitivity was detected at 3 months; no association was found at 6 months and 1 year. At 1 year, restoration size was positively associated with marginal adaptation. Despite some changes from baseline, all restorations were clinically acceptable at 1 year. Conclusion: Although flowable resin composites are advocated for occlusal restorations, it is recommended that they be limited to small and moderate-sized restorations. Keywords: clinical, flowable resin composites, marginal adaptation, marginal discoloration, occlusal, retention