Background: Osmophobia is frequent in children with migraine (20–35%) but can also occur in up to 14% of cases with tension-type headache (TTH). So far, the prognostic role of this symptom in children with primary headaches has never been evaluated. Methods: A longitudinal prospective study was conducted on 90 young patients with TTH (37 with osmophobia, 53 without osmophobia). We evaluated whether osmophobia could predict the diagnosis transformation from TTH to migraine after a 3-year follow-up. Results and Discussion: In our cases the rate of diagnosis change was significantly greater in cases with osmophobia (62%) than in those without (23%). Osmophobia persisted at a 3-year follow-up in the majority of our cases (85%) and it was found to be one of the major predictors for the development of migraine; other predictors of evolution to migraine were phonophobia, a probable rather than certain diagnosis of TTH and olfactory triggers (p < 0.05). Conclusion: Our data confirm that osmophobia has an important diagnostic and prognostic role in children with primary headaches and should be systematically investigated at diagnosis and during follow-up.