In this study, the authors examined whether the number or success of weight loss partners influences participants' outcomes in behavioral weight loss treatment. Overweight participants (n=109) assigned to an exercise intensive group in a larger trial were encouraged to invite up to 3 partners to attend treatment. Weight losses at 6, 12, and 18 months were not associated with the number of partners (0-3) but were associated with the weight loss success of the partners. Participants with at least 1 successful partner (weight loss>=10% at 6 months) lost significantly (p=.004) more weight at 6, 12, and 18 months than those with no successful partners and those without partners. Interclass correlations of weight change between participants and their partner(s) were strong at all time points (ps<.01). Support partners appear to only be beneficial in obesity treatment when partners themselves lose weight.