The role of prostanoids in nociception is well established. The headache-eliciting effects of prostaglandin E2 (PGE2) and its possible mechanisms have previously not been systematically studied in man. We hypothesized that infusion of PGE2 might induce headache and vasodilation of cranial vessels. PGE2 (0.40 μg kg-1 min-1) or saline was infused for 25 min into 11 healthy subjects in a cross-over, double-blind study. Headache intensity was scored on a verbal rating scale from 0 to 10. In addition, we recorded mean flow in the middle cerebral artery (VMCA) by transcranial Doppler and diameter of the superficial temporal artery (STA) by high-resolution ultrasonography. All 11 subjects reported headache on the PGE2 day and no subjects reported headache on the placebo day (P = 0.001). During the immediate phase (0-30 min) (P = 0.005) and the postinfusion phase (30-90 min) (P = 0.005), the area under the curve for headache score was significantly larger on the PGE2 day compared with the placebo day. PGE2 caused dilatation of the STA (23.5%; 95% CI 14.0, 37.8) and the MCA (8.3%; 95% CI 4.0, 12.6). We suggest that PGE2 induces headache by activation and sensitization of cranial perivascular sensory afferents.