&NA;Immunocomplexes (IC) in serum were analyzed in 54 patients with subarachnoid hemorrhage (SAH) from ruptured arterial aneurysms. A previous study had shown that patients with SAH and vasospasm had a significantly higher incidence of ICs in the blood than patients without vasospasm. The aim of the present study was to study how the IC content varied with time and compare this pattern with the clinical picture. Forty‐two patients presented clinical or radiological signs of cerebral vasospasm during their hospital stays, whereas 12 patients showed no such signs. The patients with vasospasm had a significantly higher amount of ICs in serum than those without vasospasm. In 37 patients with vasospasm, the changes of IC content during the 1st weeks after SAH correlated well with the clinical course. Data indicated that a high IC content preceded the onset of vasospasm and a low content preceded clinical improvement. This observation supports the idea that the presence of ICs might be the cause and not the result of vasospasm. (Neurosurgery19:767‐771, 1986)