The usefulness of a headache diary in the diagnosis of migraine and in the clarification of migraine symptoms was studied in 145 children. These children belong to a 1year age cohort of 5356 children that has been followed since birth. The children were enrolled in the present study according to their headache status in a questionnaire study at the age of 8 to 9 years, at which time 50 children had migraine, 43 had nonmigrainous headache, and 52 did not have recurrent headache. Information on their present headache status was collected with a face-to-face interview at the age of 11 to 13 years and from a headache diary after the interview. The children kept the diary for 2 to 7 months. Altogether, 72 children had migraine according to the International Headache Society criteria for migraine, either in the interview or in the diary. Eight children were diagnosed only according to the diary (11.1%). Thirty-three children had both migraine attacks and nonmigrainous headache episodes according to the diary, even though they were able to report only one type of headache episode in the interview. The duration of headache episodes was underestimated in the interview, compared to the diary in the children with migraine. Many children recognized new aura symptoms, associated symptoms, and characteristics of pain when they started to pay attention to these when filling in the diary during the follow-up period. The headache diary is useful in clarifying the features of headache attacks and in the diagnosis of headache types in children.