Medication overuse headache (MOH) is a growing problem worldwide and a challenge for clinicians and investigators. This study aims to contribute to the ongoing debate surrounding the classification of MOH. Applying the revised diagnostic criteria for MOH contained in the updated International Classification of Headache Disorders (ICHD-II), we enrolled 140 probable MOH (p-MOH) patients. They were submitted to an in-patient detoxification protocol and re-examined 2, 6 and 12 months later to confirm, or otherwise, the diagnosis of MOH and to observe the evolution of their headache. MOH diagnosis was confirmed 2 months after detoxification in 71% of patients, who reverted to an episodic headache pattern and stopped their drug overuse The overall clinical situation at 2 months closely reflected the 1-year trend. The 2-month period after drug withdrawal should be retained as a diagnostic criterion in the ICHD-II because it is useful not only as a diagnostic parameter, but also as predictor of a good outcome of 1-year drug withdrawal. In addition, the present findings point to the need for a more objective criterion to quantify headache frequency after drug withdrawal.