Objective: To evaluate the clinical efficacy of two flowable composite resins used to restore occlusal caries lesions. Tetric Flow (Vivadent) and Esthet-X Flow (Dentsply/Caulk) are composite resins with decreased filler loading (Tetric Flow: 67% filler by weight, 43% by volume; Esthet-X Flow: 61% filler by weight, 53% by volume) and lower viscosity compared to conventional composite resins. Method and Materials: Sixty-three teeth with occlusal caries were randomly assigned to calibrated practitioners who placed occlusal restorations (32 for Esthet-X Flow; 31 for Tetric Flow). After tooth preparation and caries removal, each material was inserted. A polyvinyl impression was made of each preparation and measured to record the preparation size. 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) and 3, 6, 12, and 36 months for marginal discoloration, secondary caries, anatomical form, retention, polishability, marginal adaptation, and color match. Groups were compared at 3 years and the data analyzed statistically with a .05 level of significance using generalized estimating equations, with size of restoration compared to the modified Ryge criteria. Results: Of the effects that were estimable, only color match exhibited a difference between treatment groups (P = .0467, favoring Esthet-X Flow). No other differences were observed between materials. Although no difference was observed between groups, marginal discoloration (P = .0164) and marginal adaptation (P = .0001) significantly worsened at 36 months. For polishability, a significant difference was observed at 36 months (P = .0066). No other outcomes changed significantly over time. Size of the restoration was found to be positively associated with worse outcome for color match (P = .0062 for area; P = .0439 for volume). Conclusion: Although flowable composite resins are advocated for occlusal restorations, this study suggests that they should be limited to small restorations such as preventive resin restorations having isthmus widths of one-quarter or less of intercuspal distance. Keywords: clinical, flowable composite resins, marginal adaptation, marginal discoloration, occlusal, retention