Several lines of evidence have previously suggested that platelets might play a primary or secondary role in migraine pathogenesis, and that in addition serotonin receptors on human platelets might be involved in those processes. By using [3H]-spiperone as a radioligand and ketanserin to determine the non-specific binding, the numbers of 5-HT2 serotonin receptors on platelet membrane obtained from normal healthy and migraine subjects were determined. We found a significant decrease (p < 0.02) in the maximal receptor numbers (Bmax) on platelet membrane of migraine patients (Bmax = 33.01 +/- 5.57 fmol/mg protein) when compared to the normal healthy group (Bmax = 86.59 +/- 9.09 fmol/mg protein) whereas the dissociation equilibrium constant (Kd) values (2.47 +/- 0.44 nM and 3.41 +/- 0.95 nM for the normal and migraine subjects, respectively) remained unchanged. The platelet response showed a higher degree of aggregability in migraine subjects, whereas the platelet counts were the same. This finding implies that serotonin receptors on the platelet may reflect some aspect of the pathogenesis of migraine headache.