Thirty-three patients with common migraine underwent contingent negative variation (CNV) recordings before receiving prophylactic beta-blocker treatment with either metoprolol (27 patients) or propranolol (6 patients) at mean daily dosages of 110 mg and 122 mg, respectively. After 3 months the therapeutic efficacy of the beta-blocker was assessed in each patient by means of a global severity score and compared with the initial CNV recordings. The mean clinical improvement was 62%. A significant positive correlation was found between CNV amplitude before prophylaxis and the clinical response to beta-blockers: patients with higher CNV tended to respond better to therapy. Eight of 10 patients with a CNV amplitude higher than −25 μV had a more than 50% reduction of the severity score—that is, a good or excellent response to the beta-blocking agent—whereas only 2 of 9 patients with an amplitude lower than −20 μV had a good response.