This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Headache is a common neurological symptom in clinical practice. In 1 year most of the US population will have a headache and over 5% will seek medical aid (Silberstein Silberstein, 1990). Headaches are either episodic (less than 15 headache days/month) or chronic (greater than or equal to 15 headache days/month) (Headache Classification Subcommittee of the International Headache Society, revised 2004 [ICHD-II]). Recurrent headaches (HA) can be symptoms of a chronic primary headache disorder. Primary chronic daily headache (CDH) is a syndrome characterized by HA not attributable to a secondary disorder, which last more than 4 hours/day and occur 16 or more days/month (Bigal et al, 2004; Silberstein and Lipton, 2001). Chronic daily headache is a heterogeneous disorder, affecting approximately 4% to 5% of the general population (based on global epidemiological studies; Castillo et al, 1999; Scher et al, 1998; Wang et al, 2000) and is the most common headache seen in headache specialty clinics.