Background: Anorexia nervosa (AN) is a serious mental illness with high rates of relapse and mortality. Psychiatric comorbidities are common but their impact on the prognosis is largely unknown. Objective: The aim was to investigate the influence of psychiatric comorbidity on weight gain during treatment of AN. Methods: A systematic search was performed in PubMed/MEDLINE, EMBASE and PsycINFO. Studies evaluating psychiatric comorbidity as a predictor for treatment outcome (weight gain) were included, however, comorbid alcohol/drug addiction was excluded from this review. Results: Four thousand five hundred and twenty six publications were identified from which 15 were included. The majority of the included studies had a prospective open naturalistic study design, a short-term follow-up period, and were based on small populations of primarily adolescent and adult women. Four studies indicate depression, and two obsessiveness as negative prognostic factors, whilst one study indicated moderate depression and yet another, neuroticism, as positive predictors for weight gain. Discussion: The systematic scoping review found a large number of publications whereof only a few directly described the influence of psychiatric comorbidity on weight gain in AN. Overall, studies were heterogeneous in design, purpose and outcome making comparisons difficult. Findings were divergent but depression had a negative influence on weight gain in four studies.