Background Prior studies suggest individuals with body mass index (BMI) above vs. below 60 kg/m2 show lower percentage of excess body weight loss (%EBWL) following bariatric surgery. Objectives 1. Test whether conclusions drawn about the effect of preoperative BMI on postoperative weight loss depend on the outcome measure; 2. Test for evidence of a threshold effect at BMI=60 kg/m2 and; 3. Test the effect from surgery to 12-month, relative to 12- to 36-month, follow-up. Setting Large University-affiliated hospital in New York. Methods Retrospective analyses of participants grouped according to preoperative BMI: 35–39.9 (n=232); 40–49.9 (n=1166); 50–59.9 (n=429); ≥60 (n=166). Results As anticipated, individuals with higher vs. lower preoperative BMI showed greater total body weight loss but lower %EBWL at all postoperative time points (all p’s<0.0005). However, these individuals also showed lower percentage of initial body weight loss (%IBWL) at all time points beyond 1 mo post-surgery (all p’s<0.0005). From 12- to 36-months, individuals with BMI: 35–39.9 showed 3.2±14.3 %IBWL (p<0.0001); 40–49.9 showed 1.0±8.9 %IBWL (p<0.0005); 50–59.9 showed −2.4±10.0 %IBWL (p<0.0005) and; ≥60 showed −3.6±11.5 %IBWL (p<0.0005). Overall F3,1989=20.2, p<0.0005. Conclusions Conclusions drawn about the effect of preoperative BMI may depend on the outcome measure. A dosage effect of preoperative BMI was apparent, with heavier individuals showing lower percentages of initial and excess weight loss, regardless of BMI above or below 60kg/m2. Finally, this effect was particularly apparent following the initial 12-month rapid weight loss phase, when less obese (BMI<50) individuals continued losing weight, while heavier individuals (BMI≥50) regained significant weight.