Objective.-To evaluate the efficacy of an intensive outpatient program designed to improve functioning and reduce psychological impairment in chronic headache patients. Background.-Chronic headaches, occurring 15 or more days per month, for three or more months, may arise from multiple International Classification of Headache Disorders diagnoses: Chronic Migraine, Chronic Tension Type Headache, New Daily Persistent Headache, Chronic Post Traumatic Headaches, and Medication Overuse Headache. Several interdisciplinary programs that treat patients with chronic headaches have reported decreases in headache frequency. This study sought to evaluate the effect of a 3 week interdisciplinary treatment program for patients with chronic headache disorders on headache severity, functional status, and psychological impairment. Methods.-Subjects were 379 patients admitted to an outpatient chronic headache treatment program. Assessments of headache severity, psychological status, and functional impairment were completed by 371 (97.8%) of these at the time of admission. At discharge, 340 subjects (89.7%) provided assessment data, and 152 (40.1%) provided data at 1-year follow-up. Results.-Subjects' mean ratings on a 0-10 scale for their headache pain in the prior week declined, and these improvements were maintained at follow-up. (Estimated marginal means on a 0-10 scale for Average pain: admission 6.1, discharge 3.5, follow-up 3.3; for Least pain: admission 3.2; discharge 1.5; follow-up 1.3; for Worst pain: admission 8.2; discharge 6.4; follow-up 5.7), and similar results were found for current pain (admission 4.7; discharge 2.8; follow-up 2.4): Measures of functional impairment also improved following treatment, and these gains were maintained at 12 month follow up (Estimated marginal mean Headache Impact Test-6 score: admission 66.1, discharge 55.4, follow-up 51.9; Estimated marginal mean Pain Disability Index score: admission 36.2, discharge 14.1, follow-up 11.6). As measured by the Depression, Anxiety and Stress Scale, anxiety and reactivity to stress decreased following treatment, and remained improved at follow-up (Estimated marginal mean score for Anxiety: admission 8.7, discharge 5.2, follow-up 4.4; Estimated marginal mean score for stress: admission 14.9, discharge 7.2, follow-up 7.6). Depression decreased with treatment, but while 1-year follow-up depression scores remained significantly lower than at admission, they were also significantly higher than at discharge (Estimated marginal means: admission 13.3, discharge 4.1, follow-up 6.6). Conclusions.-The study supports the efficacy of the treatment model. Limitations of the study and suggestions for future research are also discussed.