The expression of dopamine D5 receptor was investigated in peripheral blood lymphocytes of 11 migraine patients and of ten healthy control subjects using a radioligand binding technique with [3H]SCH 23390 as a ligand. [3H]SCH 23390 is a benzazepine derivative with potent antagonist properties at the dopamine D1-like receptors. [3H]SCH 23390 was specifically bound to peripheral blood lymphocytes of migraineurs and control subjects in a manner consistent with the labelling of a dopamine D5 receptor. In migraineurs a statistically significant higher density of lymphocyte dopamine D5 receptor compared with controls was noticeable, whereas the affinity of the radioligand was unchanged. The increased density of dopamine D5 receptor in peripheral blood lymphocytes may reflect the dopaminergic hypersensitivity displayed by migraineurs and may represent a relatively simple and reliable peripheral marker of altered dopaminergic function.