OBJECTIVE:To better understand the role interpersonal problems play in response to two treatments for generalized anxiety disorder (GAD); an acceptance-based behavior therapy (ABBT) and applied relaxation (AR), and to examine how the development of mindfulness may be related to change in interpersonal problems over treatment and at follow-up. METHOD: Eighty-one individuals diagnosed with GAD (65.4% female, 80.2% identified as white, average age 32.92) were randomized to receive 16 sessions of either ABBT or AR. GAD severity, interpersonal problems, and mindfulness were measured at pre-treatment, post-treatment, 6-month follow-up, and 12-month follow-up. RESULTS: Mixed effect regression models did not reveal any significant effects of pre-treatment interpersonal problems on GAD severity over treatment. After controlling for post-treatment GAD severity, remaining post-treatment interpersonal problems predicted 6- but not 12-month GAD severity. Participants in both conditions experienced a large decrease in interpersonal problems over treatment. Increases in mindfulness over treatment and through follow-up were associated with decreases in interpersonal problems, even when accounting for reductions in overall GAD severity. CONCLUSIONS: Interpersonal problems may be an important target of treatment in GAD, even if pre-treatment interpersonal problems are not predictive of outcome. Developing mindfulness in individuals with GAD may help ameliorate interpersonal difficulties among this population.