Objective Patients with anorexia nervosa (AN) exhibit high rates of psychiatric comorbidity. To disentangle the effects of duration of illness on comorbid psychiatric symptoms, we investigated the rates of comorbid psychiatric disorders, suicidality and self‐harm behaviour in adolescent patients with a first onset of AN. Methods In adolescent females (n = 148) with a first onset of AN, body mass index, psychiatric comorbidity (according to DSM‐IV), depressive symptoms, suicidality and self‐injurious behaviour were assessed. Results Seventy patients (47.3%) met the criteria for at least one comorbid psychiatric disorder. The binge‐purging subtype was associated with increased rates of psychiatric comorbidity, suicidality and self‐injurious behaviour. The severity of eating disorder‐specific psychopathology influenced current psychiatric comorbidity and suicidal ideation. Conclusion Prevalence rates of comorbid psychiatric disorders and suicidal ideation are considerably lower among adolescents with AN compared with adults. An early and careful assessment, along with adequate treatment of the eating disorder, might prevent the development of severe psychiatric comorbidities. Copyright © 2013 John Wiley & Sons, Ltd and Eating Disorders Association.