SYNOPSIS,Chronic paroxysmal hemicrania (CPH) is a cluster headache variant, characterized by daily, frequent, unilateral, excruciatingly severe but rather short lasting headache attacks, which will not surface during chronic indomethacin medication. Three of five patients with an established diagnosis of CPH were able to precipitate attacks by head flexion (or rotation), whereas this ability was not detected in any of 40 patients with regular cluster headache. Attacks with associated unilateral tearing, conjunctival injection, slight miosis, ptosis and headache may start 4–15 second after beginning of head flexion. Precipitated and spontaneous attacks seem identical both clinically and as far as the immediate increment in corneal indentation pulse (CIP) amplitudes and intraocular pressure are concerned.There are various alternative explanations for the underlying mechanism, the most plausible of which concerns sympathetic nerve involvemen