The mechanisms of the postulated "sterile" inflammation in migraine were studied utilizing flow cytometry (intercellular adhesion molecule 1, ICAM-1; interleukin-1 receptor, IL-1R) and enzyme-linked immunosorbent assay (soluble intercellular adhesion molecule 1, sICAM-1; interleukin-4, IL-4). Twenty patients suffering from migraine without aura, 20 healthy subjects, and 10 patients suffering from episodic tension headache were selected. All of the migraine patients were studied during a migraine crisis experimentally induced by the administration of isosorbide dinitrate (a nitric oxide donor), and 10 out the 20 were also studied during a spontaneous migraine attack. A sharp decrease in the expression of ICAM-1 (F=5.09, p<0.001 and F=2.46, p<0.05, respectively), sICAM-1 1 (F=6.21, p<0.0001 and F=3.99, p<0.007, respectively) and serum IL-4 (F=6.23, p<0.001 and F=3.64, p<0.01, respectively) were observed in experimentally induced and spontaneous migraine attacks. There was no change with respect to IL-IR 1 receptor expression values. The two control groups, tested with the same experimental procedure, showed no changes in ICAM-1 and IL-1R or in in sICAM-1 and IL-4. Our data suggest that migraine patients are more sensitive to exogenous NO than controls. In addition, our results indicate that experimental migraine crisis, induced by an NO donor, is mediated by the inhibition of IL-4 and subsequently of ICAM-1. It is likely that the described ICAM-1 downregulation inhibits during a migraine attack the critical step of transendothelial migration into the cerebral tissues of activated leukocytes, as proposed in the "sterile inflammation" hypothesis.