Serum complement component concentrations and immune complexes were studied in 47 patients withcluster headache, 32 with common migraine, 12 with classical migraine and two with possible milk-allergicmigraine. No significant differences were found between the groups and two control populations: 63neurological patients without migraine and 30 normal healthy volunteers. The findings were similar both inand out of active periods of cluster headache and before and during migraine attacks. Bovine serumalbumin was detectable in the serum of the two cases postulated to have milk-allergic migraine afteringestion of milk, but in neither case was an attack of migraine precipitated. Our results do not supportimplication of the complement system in either type of headache.