Background: Underweight eating disorders (EDs) are notoriously difficult to treat, although a growing evidence base suggests that outpatient cognitive behaviour therapy for EDs (CBT-ED) can be effective for a large proportion of individuals. Aims: To investigate the effectiveness of CBT-ED for underweight EDs in a ‘real-world’ settings. Method: Sixty-three adults with underweight EDs (anorexia nervosa or atypical anorexia nervosa) began outpatient CBT-ED in a National Health Service setting. Results: Fifty-four per cent completed treatment, for whom significant changes were observed on measures of ED symptoms, psychological distress and psychosocial impairment. There was also a large effect on body weight at end-of-treatment. Conclusions: The results suggest that good outcomes can be achieved by the majority of those who complete treatment, although treatment non-completion remains a significant barrier to recovery. Future studies should focus on improving treatment retention, as evidence suggests that CBT-ED in ‘real-world’ settings is effective.