Background. - The clinical features of cluster headache (CH)disclose some vascular changes in the symptomatic region, but few instrumental studies have assessed orbital hemodynamics in patients with this disorder. Methods. - Orbital blood flow reactivity elicited by Valsalva maneuver (VM) was studied with ophthalmic artery eco- Doppler in 16 patients (14 men and 2 women; mean age: 41.2) suffering from episodic CH and in 18 healthy controls. Patients were examined twice: first, in a cluster period (between pain attacks), and second, in a remission period. Each time peak- systolic and end- diastolic flow velocities were recorded in both ophthalmic arteries at rest and during all phases of VM. Results. - Valsalva phase IV was consistently associated with an increment of blood flow velocities through the ophthalmic arteries. Unlike controls, patients showed an asymmetric vascular reactivity. In the cluster period peak- systolic flow velocity increments were lower on the symptomatic side than on the asymptomatic side (14.1% vs. 34.4% ; P < .001), while in remission end- diastolic flow velocity increments were higher in the previously symptomatic orbit (129% vs. 72.9% ; P < .05). Vascular reactivity on the asymptomatic side was always similar to that of healthy controls. Conclusions. - In episodic CH, the symptomatic orbit shows an abnormal vascular reactivity. During the cluster period, basal vasodilation and hyperemia could preclude it from admitting a much greater amount of blood at the end of Valsalva. During remission, there might be some latent vascular changes that lead to supersensitive vasodilator responses and/or opening of arteriovenous shunts under certain circumstances such as Valsalva. These phenomena could be relevant in the pathophysiology of CH. Background. The clinical features of cluster headache (CH) disclose some vascular changes in the symptomatic region, but few instrumental studies have measured orbital hemodynamics in patients with this disorder. Methods. - Orbital blood flow reactivity elicited by Valsalva maneuver (VM) was Patients with ovhlmic artery eco-Doppler in 16 patients (14 men and 2 women; mean age: 41.2) suffering from episodic CH and in 18 healthy controls. Patients were examined twice: first, in a cluster period (between pain attacks), and second, in a remission period. Each time peak-systolic and end-diastolic flow velocities were recorded in both ophthalmic arteries at rest and during all phases of VM. Valsalva phase IV was consistently associated with an increment of blood flow velocities through The ophthalmic arteries. Unlike controls, patients showed an asymmetric vascular reactivity. In the cluster period peak-systolic flow velocity increments were lower on the symptomatic side th an on the asymptomatic side (14.1% vs. 34.4%; P <.001) while in remission end-diastolic flow velocity increments were higher in the previously symptomatic orbit (129% vs. 72.9%; P <.05) During the cluster period, basal vasodilation and hyperemia could preclude it from admitting a much greater amount of blood at the end of Valsalva. During remission, there might be some latent vascular changes that lead to supersensitive vasodilator responses and / or opening of arteriovenous shunts under certain circumstances such as Valsalva. These phenomena could be relevant in the pathophysiology of CH.