High rates of comorbidity are found among eating disorder (ED) patients, which may negatively affect treatment outcome and prognosis. However, there is a shortage of studies in Spain using clinician administered interviews to assess rates of comorbidity among these patients, particularly in adolescents. This study aimed to evaluate Axis I psychiatric diagnoses in adolescent patients with an ED and to compare them with patients with a distinct disorder with adolescent onset, substance use disorder (SUD) patients. Considering that maternal psychological distress is another factor involved in ED prognosis, a secondary aim was to examine the relationship between patient's psychological variables and niaternal distress (depression and anxiety). The cross-sectional study included 50 ED patients, 48 SUD patients, and their mothers. More than half of the patients received a diagnosis for a comorbid disorder. Internalizing problems were more common among EDs and externalizing disorders were the most common comorbidities among SUDs, similar to findings from other Countries. Maternal distress was associated with higher levels of depression and symptom severity in patients. No differences in distress were found between mothers of patients with a comorbid diagnosis and those without. Elevated anxiety or depression in mothers did not increase the likelihood of patients having a particular primary diagnosis. In short, while both ED and SUD patients presented high rates of comorbidity, the types of comorbid diagnoses were specific to each group. Assessing for the presence of comorbid disorders and targeting maternal psychological distress may guide treatment interventions and improve patient prognosis. (C) 2016 Elsevier Ltd. All rights reserved.