Aims: To prospectively assess the incidence and etiology (ie, primary vs symptomatic) of headache in women during the first month postdelivery, with particular emphasis on the type of presentation as a clue for identifying potentially harmful etiologies. A secondary aim was to evaluate the relative frequency of migraine- vs tension-type headache in cases of primary headache. Methods: A total of 900 consecutive women were enrolled in the study and examined within 3 days of delivery, both clinically and with transcranial color-coded sonography (TCCS). During the course of follow-up, all subjects presenting with headache suspected of being secondary to intracranial pathology underwent a complete clinical and instrumental assessment with TCCS and magnetic resonance imaging (MRI) and angiography. A telephone interview was administered to all subjects 1 month after delivery. Two-tailed t test, Mann-Whitney test, Pearson chi-square test, and multiple logistic regression were used to analyze the data. Results: At the end of the follow-up period, 241 women (26.8% of the sample) reported at least one headache attack. In 88 of these 241 cases (9.8%), the headache attack occurred soon after delivery and was already recorded at the first visit. Thunderclap headache occurred in 34 (3.8%) of the subjects. In all but one of these subjects, the course was spontaneously benign. None of the recorded variables allowed discrimination of the subjects with thunderclap headache from those without headache. Three subjects had thunderclap headache following dural anesthesia, and one subject was found to have reversible cerebral vasoconstriction syndrome. Headache with gradual onset was recorded in 207 subjects (23%). Three of these subjects fulfilled the criteria for pre-eclampsia, and 13 had postural headache after dural anesthesia. Migraine history and urinary protein were independent predictors of gradual onset headache, and migraine history and parity were significant independent predictors of pulsating pain with gradual onset headache. Conclusion: Headache appeared early in the first days postdelivery, and its incidence increased in the first month thereafter. Predictors were different according to whether the headache had a gradual onset or a thunderclap presentation. Primary headache accounted for the overwhelming majority of the recorded cases. Keywords: migraine, puerperium, reversible cerebral vasoconstriction syndrome, thunderclap headache