PURPOSE: Many patients presenting with headache also complain of constipation; the relationship between these two symptoms has not been explored in detail. The aim of this study was to investigate the association between primary headache and constipation. METHODS: This retrospective study included all children who attended the Inje University Sanggye Paik Hospital complaining of headache, and who had been followed up for at least 100 days. Patients were divided into 2 groups: group A, in whom the headache improved after treatment for constipation, and group B, in whom headache was not associated with constipation. RESULTS: Of the 96 patients with primary headache, 24 (25.0%) also had constipation (group A). All 24 received treatment for constipation. Follow-up revealed an improvement in both headache and constipation in all patients. Group B contained the remaining 72 children. Comparison of groups A and B indicated a significant difference in sex ratio (P=0.009, chi-square test). Patients with probable tension-type headache were more likely to be in Group A (P=0.006, chi-square test). CONCLUSION: Resolution of constipation improves headache in many patients diagnosed with primary headache, especially those with probable tension-type headache. We suggest that either constipation plays a key role in triggering headache, or that both constipation and headache share a common pathophysiology.