Although chronic daily headache is regarded as a syndrome encountered in headache clinics, clinical characteristics have only rarely been studied and the condition has not been documented in Thailand. To investigate the prevalence as well as clinical features of chronic daily headache in Thai patients, 220 patients visiting Chulalongkorn Headache Clinic were examined. Sixty cases (27.3%) were diagnosed as suffering from chronic daily headache (male to female ratio, 1:5.7). The average age of these patients was 32.7 +/- 9.6 years. Based on the International Headache Society (IHS) criteria, 30% of patients with chronic daily headache could be diagnosed as suffering from migraine and 36.7% from chronic tension-type headache, whereas the remainder had combined features of both headache types and were not classifiable. Diffuse steady pain was the most common headache type reported (65%), however, associated features characteristic of migraine were often noted. These included photophobia (70%), phonophobia (56.7%) and nausea (43%). Thirty-four cases (56.7%) reported that their headache could be aggravated by stress. Daily use of analgesics was reported in 58.3% of cases. We concluded that chronic daily headache is a common problem. Although the mechanism has not been fully clarified, the prevalence of associated psychological factors and analgesic overuse imply their involvement in the pathogenesis of this condition. The criteria of the IHS are not entirely suitable for diagnosis and classification of this disorder, and modification of this classification system is needed.