The onset of post-traumatic headache (PTC) occurs in the first seven days after trauma, according to the International Headache Society (IHS) classification. The objective of this study was to evaluate the several forms of headache that appear after mild head injury (HI) and time interval between the HI and the onset of pain. We evaluated 41 patients with diagnosis of mild HI following the IHS criteria. Migraine without aura and the chronic tension-type headache were the most prevalent groups, occurring in 16 (39%) and 14 (34.1%) patients respectively. The time interval between HI and the onset of headache was less than seven days in 20 patients (48.7%) and longer than 30 days in 10 (24.3%) patients. The results suggest that PTC may arise after a period longer than is accepted at the present by the IHS.