Background: To examine the prevalence and characteristics of acute posttraumatic headache (APTH) attributed to mild head injury within a prospective, observational study design. Methods: We recruited 100 patients with acute mild head injury as defined in the International Classification of Headache Disorders, 2nd Edition (ICHD-2) presenting to the department of trauma surgery at the Medical University of Vienna. Patients underwent a detailed telephone interview between days 7 and 10 and between days 90 and 100 after the injury. Results: The prevalence of APTH was 66%. APTH had occurred within 24 hours after the trauma in 78% and lasted for a median of 3.0 days. Headache was unilateral in 45%. Aggravation by physical activity, nausea and photo-/phonophobia was present in 49%, 42% and 55%, respectively. The prevalence of APTH was related to conditions of chronic pain (excluding headache), pre-existing episodic headache, number of posttraumatic symptoms, anxiety and depression. At follow-up at 90–100 days, posttraumatic headache had abated in all patients. Conclusions: APTH attributed to mild head injury is a common but self-limiting condition frequently showing migrainous features. Participants with chronic pain other than headache, pre-existing headache and affective disorders are at higher risk of developing APTH. None of the patients developed chronic posttraumatic headache.