Thirty-two cluster headache patients and healthy controls (n = 16-20 for the various tests) were examined by means of a Whitaker pupillometer during pain-free intervals. Eye drops of the sympathomimetic agents tyramine, hydroxyamphetamine, and phenylephrine were instilled into the conjunctival sacs on separate occasions, and pupillary diameters recorded at standard time intervals. The mydriatic responses of the two pupils were compared. A moderate, but statistically significant, basal relative miosis was found on the pain side in cluster headache. The symptomatic-side pupils were less responsive than their counterparts when stimulated with tyramine and hydroxyamphetamine, the difference being statistically significant for the OH-amphetamine test. With the phenylephrine test, however, the mydriasis on the symptomatic side significantly exceeded that of the contralateral pupil. This pattern of reactions does not quite correspond to those of "ordinary" Horner's syndrome (1st, 2nd, and 3rd neuron lesion). There are, however, gross similarities with the recently reported pattern in central sympathetic neuron dysfunction. In cluster headache there is probably a "Horner-like picture" rather than a proper Horner's syndrome.