Frontal sympathetic skin responses (F-SSRs) were recorded to investigate sympathetic nervous system activity in migraine headache (MH). Thirty-five patients with unilateral MH and 10 healthy volunteers were studied by evoking bilateral F-SSRs with electrical stimulation of the median nerve in attack, post-attack and interictal periods. The mean latencies were longer and the maximum amplitudes were smaller on the symptomatic side compared with the asymptomatic side (P < 0.05 for both amplitude and latency) in attack and in interictal periods. In five patients, F-SSRs were absent bilaterally, in four patients the responses were absent only on the symptomatic side during the attack period. In the post-attack period, F-SSRs on the symptomatic side had higher amplitudes and shorter latencies compared with the asymptomatic side (P < 0.01 for both amplitude and latency). There is an asymmetric sympathetic hypofunction on the symptomatic side in attack and interictal periods, whereas there is a hyperfunction in the post-attack period.