Background: Co-morbidity between Type 1 Diabetes Mellitus (T1DM) and eating disorders (ED) has been previously described; however the effect of this illness on the outcomes for conventional ED treatments has not been previously investigated. This study aims to compare clinical, psychopathological and personality features between two samples of ED individuals: those with comorbid T1DM and those without (No-DM); and to identify differences in treatment outcomes between the groups. Methods: This study compares treatment outcome, dropouts, ED psychopathology and personality characteristics for 20 individuals with ED and T1DM and 20 ED patients without diabetes, matched for diagnostic and treatment type. Results: The study found higher dropout rates from therapy in individuals with T1DM and worse treatment outcome in spite of having no significant differences in eating disorder psychopathology, although individuals with T1DM report misusing insulin. Conclusions: The low levels of motivation to change, and insulin abuse in T1DM patients, may suggest that treatment for patients with ED and T1DM should consider the individual's personality and role of insulin abuse when determining the appropriate intervention.