Because of an important burden of disease, obesity is a major public health challenge in the twenty-first century. Where medico-psychological management has shown its limitations, bariatric surgery is now acknowledged as the most efficient therapy potentially offered to severely obese patients. Among other options, Roux-en-Y gastric bypass (RYGBP) is the most frequently performed procedure. The objective of this review is to systematically evaluate the effect of the Roux- (alimentary) limb length on postoperative weight loss after RYGBP in severely obese patients. MEDLINE, EMBASE and the Cochrane Central Register of Controlled Trials (CENTRAL) were searched using terms related to Roux-limb, gastric bypass and obesity. To be included, studies had to be either randomized controlled trials, quasi-randomized controlled trials or prospective cohort studies comparing a shorter to a longer Roux-limb. Studies were critically appraised with regard to methodological components. Eight studies were reviewed. Variations in methodology, operation design and outcome assessment among studies caused considerable clinical heterogeneity, preventing us from performing a meta-analysis. The overall quality was questionable, owing to lack of rigor in methodological components reporting. Results were heterogeneous, but we identified a trend supporting that the construction of a longer Roux-limb is more efficient in super obese patients. This review suggests that the tailoring of a longer Roux-limb might only be efficient in super obese patients. The overall limited quality of the included studies prompts to call for improvement in trial design in surgery.