We performed serial measurements of 8-iso-prostaglandin F2alpha (8-iso-PGF2alpha), a non-enzymatic derived arachidonyl peroxide, in the cerebrospinal fluid (CSF) of 34 patients with subarachnoid hemorrhage (SAH). Patients were treated with open or endovascular surgery within 48 h of onset. Delayed cerebral vasospasm was verified by the presence of a low-density area on CT scan indicating focal cerebral infarction occurring after symptomatic delayed vasospasm. Concentrations of 8-iso-PGF2alpha in the CSF of 15 patients exhibiting delayed cerebral vasospasm were compared with those of 19 patients who did not exhibit vasospasm. The concentrations of 8-iso-PGF2alpha in the CSF of patients showing vasospasm were 42.4+/-37.1 pg/ml (mean+/-S.D., n=12) on Days 0-2, 66.4+/-41.0 pg/ml (n=14) on Days 3-5, 118.5+/-89.9 pg/ml (n=15) on Days 6-8, 86.2+/-70.2 pg/ml (n=11) on Days 9-11, 48.8+/-31.8 pg/ml (n=10) on Days 12-14, 27.8+/-20.1 pg/ml (n=7) after Day 20, while the concentrations in patients not showing vasospasm were 24.8+/-12.0 pg/ml (n=18) on Days 0-2, 25.7+/-15.2 pg/ml (n=19) on Days 3-5, 47.5+/-52.3 pg/ml (n=18) on Days 6-8, 56.7+/-72.0 pg/ml (n=13) on Days 9-11, 34.2+/-53.1 pg/ml (n=15) on Days 12-14, 20.1+/-18.2 pg/ml (n=10) after Day 20. CSF concentrations of 8-iso-PGF2alpha on Days 3-5 and Days 6-8 were significantly higher in patients showing vasospasm as compared to patients not showing vasospasm. CSF levels of 8-iso-PGF2alpha in patients showing vasospasm gradually increased in the days after onset of SAH and peaked on Days 6-8. Levels returned to normal after Day 20. These values on Days 3-5, Days 6-8, and Days 9-11 were significantly higher than the value after Day 20. Considering these data and the biological activities of 8-iso-PGF2alpha, such as development of inflammation, membrane perturbation and vasoconstriction, we conclude that 8-iso-PGF2alpha may play a role in delayed cerebral vasospasm after SAH.