The present study examined the sensitivity to light of melatonin (MLT) secretion in familial migraine during a headache-free interval. Twelve female patients and 12 healthy controls were included in the trial. All subjects were studied twice. In each session, light exposure (300 lx) or placebo was randomly administered for 30 min between 00.30 and 01.00 h. Blood was sampled hourly between 20.00 and 24.00 h, and 02.00 and 04.00 h and every 15 min between 00.30 and 01.30 h. Plasma MLT levels were determined by radioimmunoassay. MLT suppression was more marked in the migraine group than in the control group [difference of area under curve (DeltaAUC)=-53.8 +/- 16.2 vs. 18.5 +/- 12.7 pg/h/ml, P<0.005; maximum of MLT suppression (Delta)=-35.7 +/- 10.2 vs. - 6.7 +/- 5.8 pg/ml, P<0.05]. These findings show a clear hypersensitivity to light in young female migraineurs during the headache-free period.