Objective: Patients with bulimia nervosa (BN) often gain weight during treatment, which potentially poses a threat to treatment adherence. Although weight suppression has been found to be a predictor of weight gain in persons with BN, research about the nature of weight changes during treatment and other predictors thereof is scarce.Method: The current study examined weight suppression as well as self-reported bingeing severity and purging frequency at admission as predictors of weight change during inpatient treatment in 746 persons with BN (95% female).Results: Body mass index (BMI) increased linearly across treatment weeks. Higher weight suppression predicted larger weight gain, particularly in those with a relatively low BMI at admission. More frequent purging and less severe bingeing predicted larger weight gain but high bingeing severity in combination with infrequent purging attenuated this effect.Conclusions: Results replicate that those with high weight suppression are at higher risk for gaining weight during BN treatment but extend these findings in that this effect additionally depends on current BMI, similar to findings reported in persons with anorexia nervosa. They further demonstrate that the core features of BN—bingeing and purging—also predict weight change both separately and interactively and may, therefore, be considered in psychoeducation and therapy planning.