After weight loss surgery (WLS), psychosocial functioning, including the quality of social relationships, generally improves, but for a minority, relationships worsen. We examined how changes in relationship stability and quality from pre-to post-WLS relate to long-term weight loss outcomes. Postoperative patients (N = 361) completed surveys which queried relationship changes and weight loss. The sample was 95.9% Caucasian, 80.1% female, averaged 7.7 years post-WLS, with a mean age at surgery of 47.7 years (range 21-72); 87.3% had a Roux-en-Y gastric bypass. Four relationship status groups were created: Not in a relationship at surgery or follow-up (No-Rel, n = 66; 18.2%); Post-WLS relationship only (New-Rel, n = 23; 6%); Pre-WLS relationship only (Lost-Rel, n = 17; 5%); and Pre-Post Relationship (Maintainer, n = 255; 70.6%). Current BMI was 34.5 for No-Rel; 40.5 for New-Rel; 37.4 for Lost-Rel; 33.3 for Maintainers (p < .05 for Maintainers and No-Rel vs. New-Rel). These same group differences were significant for weight loss, which was not associated with gender, time since surgery, or age at time of surgery, but was associated with pre-WLS BMI (lower pre-WLS BMI was associated with greater %EWL). Analyses were repeated with pre-WLS BMI as a covariate; group differences remained significant [F (3, 355) = 3.09, p = .03], as did pre-WLS BMI, [F (1, 355) = 9.12, p = .003]. Among Maintainers, relationship quality was associated with weight loss outcomes: those with improved relationships post-WLS had significantly greater % EWL [F (2, 234)= 15.82, p < 0.000; p < .05 for Improved N (Stayed Same = Got Worse)]. Findings support the importance of assessing relationship stability and quality in pre-WLS candidates, as healthy and stable relationships may support improved long-term outcomes. Interventions to improve relationships pre- and post-WLS may increase both quality of life and weight loss outcomes. (C) 2014 Elsevier Ltd. All rights reserved.