According to Sjaastad, the pain in cervicogenic headache, a form not recognized by the IHS, is long lasting and always side-locked unilateral. The frequency of side-locked unilateral pain (defined here as no change in side from onset) and other characteristics of cervicogenic headache were investigated in 300 outpatients using information collected on standard forms in structured interviews. Three hundred seventy-four headaches diagnosed according to IHS criteria were identified. Three hundred forty-eight of these headaches were long-lasting (duration of more than 4 hours); migraine (65%) followed by tension-type headache (25%) were the commonest forms. Side-locked unilaterality was present in 29% (101 of 348), and occurred most frequently in migrainous disorders not fulfilling the criteria (25 of 56, 44.6%). This group differed significantly from the other migraine conditions for longer pain duration (P<0.02) and less frequent nausea, vomiting, photophobia, phonophobia (P<0.0001), and aggravation by physical activity (P<0.02). With these characteristics, this group resembled cervicogenic headache. However, in none of these patients was pain triggered by head or neck movements, and the frequency of head or neck trauma did not differ from other headaches. A more precise definition of clinical criteria for cervicogenic headache vs migraine is, therefore, required.