Urinary excretion of histamine, as well as histaminuria following intravenous L-histidine loading, were studied in patients with so-called vascular headache. It was found that urinary excretion of histamine was increased on one or more occasions in 7 of 22 patients with cluster headache. The excretion was significantly higher on attack days than on attack free days. With migraine, increased excretion was found in 5 of 31 patients on days of an attack, whereas the corresponding figure for headache free days was 7 of 24 patients. Three patients showed increased histamine excretion during, as well as between, attacks. The excretion on attack days was not significantly different from that on attack free days. In cluster headache patients, L-histdine administration on attack days did not indicate that an increased histamine formation took place under such circumstances. The underlying mechanism behind the increased histamine output with cluster headache may be increased formation or liberation or altered catabolism. Histamine is more likely to be a consequence than the cause of an attack of cluster headache.