Objectives.- To assess headache- related disability in chronic migraine wit h medication overuse (CMO), as defined by Silber- stein and Lipton (1996, 2001) in comparison to that caused by migraine without aura, investigating similariti es and differences in disability patterns in these two conditions. Background.- It is well known that migraine has a marked impact on daily activities. Less in formation is available on the functional consequences of chronic migraine. Metho ds.- This was a prospective study on 247 patients with CMO and 264 with migrain e without aura. All completed the validated Italian version of the Migraine Disa bility Assessment questionnaire (MIDAS). The MIDAS total score and scores of eac h question (investigating total or partial inability to perform different daily activities)- were evaluated, and compared in the two groups using the Mann- Wh itney rank sum test. We also evaluated the number of missed days (question 1) an d days with significantly reduced effectiveness at work (question 2) and in nonw ork activities (questions 3, 4, and 5), using descriptive statistics. Results.- MIDAS total score in the CMO group was rather high (mean 84.1, median 70) and s ignificantly higher than in the migraine group (mean 23.4, median 19). CMO patie nts also reported higher scores than migraine patients in MIDAS questions invest igating disability in different domains, the differences being highly significan t for questions 3, 4, and 5. In both CMO and migraine the number of days with to tal or significant disability in nonwork activities was higher than the number o f days with total or significant disability in work activities, and days spent a t work with significantly reduced effectiveness were more than work days missed. Conclusions.- Our findings show that CMO has a profound effect on patients’ daily functioning. They suggest that differences between CMO and migraine are ma inly quantitative, while the two groups are similar in terms of disability patte rn. These results are consistent with the fact that CMO may be an evolution of e pisodic migraine, characterized by increase in headache frequency and more evide nt disability in performing different daily duties. Objectives .- To assess headache-related disability in chronic migraine wit h medication overuse (CMO), as defined by Silber- stein and Lipton (1996, 2001) in comparison by that migraine without aura, investigating similariti es and differences in disability Patterns in these two conditions. Background It is well known that migraine has a marked impact on daily activities. Lesson formation is available on the functional consequences of chronic migraine. Metho ds.- This was a prospective study on 247 patients with CMO and 264 with migrain e without aura. All completed the validated Italian version of the Migraine Disabeility Assessment questionnaire (MIDAS). The MIDAS total score and scores of eac h question (investigating total or partial inability to perform different daily activities) - were , and compared in the two groups using the Mann-Wh itney rank sum test. We also evaluated the number of missed days (question 1) an d days with significantly reduced effecti veness at work (question 2) and in nonw ork activities (questions 3, 4, and 5), using descriptive statistics. Results- MIDAS total score in the CMO group was rather high (mean 84.1, median 70) and s ignificantly higher than in the migraine group (mean 23.4, median 19). CMO patie nts also reported higher scores than migraine patients in MIDAS questions invest igating disability in different domains, the differences being highly significan t for questions 3, 4, and 5. In both CMO and migraine the number of days with to tal or significant disability in nonwork activities was higher than the number of days with total or significant disability in work activities, and days spent at work with significant reduced effectiveness were more than work days missed. Conclusions. - Our findings show that CMO has a profound effect on patients’ daily functioning. They suggest that differences between CMO and migraine are in quantitative, while the two groups are similar in terms of disability. patte rn. These results are consistent with the fact that CMO may be an evolution of e pisodic migraine, characterized by increase in headache frequency and more evide nt disability in performing different daily duties.