Prostaglandins (PGs) are involved in nociception and mast cell degranulation. Prostaglandin D₂ (PGD₂) is a vasodilatator released during mast cell degranulation. The headache-eliciting effect of PGD₂ has not been studied in man. Twelve healthy volunteers were randomly allocated to receive intravenous infusion of 384 ng/kg/min PGD₂ over 25 min in a placebo-controlled, double-blind cross-over study. We recorded headache intensity and associated symptoms, velocity in the middle cerebral artery (V(MCA)) and diameter of the superficial temporal artery (STA) and radial artery (RA) using ultrasonography. In the period 0-14 h, 11 subjects reported headache on PGD₂ compared to one subject on placebo (P = 0.002). During the in-hospital phase (0-120 min), the area under the headache curve was larger on PGD₂ compared to placebo (P < 0.05). Median peak headache, 1 (0-1), occurred 10 min after start of PGD(2) infusion. There was no difference in incidence of headache in the post-hospital phase between PGD₂ (n = 3) and placebo (n = 1). There was a decrease in V(MCA) (P < 0.001), increase in STA (P < 0.001) and RA (P < 0.006) diameter during PGD₂ infusion compared to placebo. Peak decrease in V(MCA) was 28.3% after 10 min and peak increase in STA was 55.7% after 20 min on the PGD₂ day. The present study shows that PGD₂ is a very strong vasodilator of MCA, STA and RA, but causes only mild headache.