Fifty-three patients with cluster headache, mean age 42.6 years, were examined by means of pupillometry. Pharmacological stimulation was carried out by the instillation of eye drops; the sympathomimetic agents hydroxyamphetamine (a norepinephrine releaser) and phenylephrine (an agent acting directly on the postsynaptic receptors) were applied. Pupillary dilatation was measured at set time intervals, comparing the responses of the symptomatic and non-symptomatic sides. The material was divided into groups according to the degree of basal anisocoria. The subgroup with the most pronounced basal miosis of the symptomatic side demonstrated a uniform pattern of deficient symptomatic side dilatation after OH-amphetamine and supersensitivity to phenylephrine. The other groups demonstrated the same general pattern, but to a far lesser degree. In cluster headache, the extent of nonresponsiveness to OH-amphetamine and of phenylephrine supersensitivity on the symptomatic side thus, at least partly, seems to be a function of the degree of anisocoria. The response pattern in cluster headache seems to differ from that of 3rd neuron Horner's syndrome with an anisocoria of the same extent.