IntroductionThe duration of untreated illness (DUI), that is, the interval between the onset of anorexia nervosa (AN) symptoms and start of specialized treatments, has a strong influence on the prognosis.ObjectiveTo quantify modifiable predictors of the DUI and to derive recommendations for secondary prevention strategies.MethodsWithin a multicenter, multi‐informant study, DUI was assessed in interviews with patients undergoing first specialized AN treatment. Modifiable factors were assessed perspectives of AN‐patients, their relatives, and primary care practitioners [PCPs]) with the FABIANA‐checklist (Facilitators and barriers in anorexia nervosa treatment initiation). The effect of FABIANA‐items on the DUI for each perspective was calculated using Cox Regression (control variables: age, eating disorder pathology, health care status, migration background, body mass index [BMI]).ResultsWe included data from N = 125 female patients with AN (72 adults, 53 adolescents, Mage = 19.2 years, SD = 4.2, MBMI = 15.7 kg/m2, SD = 1.9), N = 89 relatives (81.8% female, 18.2% male, Mage = 46.0 years, SD = 11.0) and N = 40 PCPs (Mage = 49.7 years, SD = 9.0). Average DUI was 12.0 months. Watching or reading articles about the successful treatment of other individuals with AN (patients' perspective) and regular appointments with a PCP (PCPs' perspective) were related to a shorter DUI (HR = 0.145, p = .046/ HR = 0.395, p = .018). Patients whose relatives rated that PCPs trivialized patients' difficulties had a longer DUI (HR = −0.147, p = .037). PCPs and relatives rated PCPs' competence higher than patients did.DiscussionIt is recommended (a) to incorporate treatment success stories in prevention strategies, (b) to inform PCPs about potential benefits of regular appointments during the transition to specialized care, and (c) to train PCPs in dealing with patients' complaints.Public SignificanceMany individuals with AN seek treatment very late. Our study shows that a promising approach to facilitate earlier AN treatment is to inform patients about successful treatments of affected peers, to foster regular appointments with a PCP and, to motivate these PCPs to take individuals' with AN difficulties seriously. Thus, our study provides important suggestions for interventions that aim to improve early treatment in AN.