Nitric oxide (NO) may play a key role in migraine and other vascular headaches since glyceryl trinitrate (a donor of NO) and histamine (which probably activates endothelial NO formation) both cause a pulsating dose-dependent headache with several migrainous characteristics. At relatively high doses of glyceryl trinitrate, migraine sufferers develop stronger and more migraine-like headaches and more pronounced cerebral arterial dilatation than controls. After the infusion of glyceryl trinitrate, non-migraineurs remain headache-free while migraineurs develop a migraine-like attack. In this review, Jes Olesen, Lars Thomsen and Helle Iversen suggest that migraine may be caused by increased amounts and/or affinity of an enzyme in the NO-triggered cascade of reactions. NO may also be involved in the pathogenesis of other vascular headaches.