Primary chronic daily headache can be subdivided into transformed migraine, chronic tension-type headache, hemicrania continua, and new daily persistent headache. We proposed and tested criteria in 150 consecutive outpatients with chronic daily headache. Based on preliminary analysis, we revised the criteria for transformed migraine. Using the International Headache Society criteria, 43% of the patients could not be classified; using our old criteria, 25% could not be classified; however, using our new criteria, we were able to classify 100%. Seventy-eight percent had transformed migraine, 15.3% had chronic tension-type headache, and 6.7% had other headache disorders.