In a double-blind crossover trial of two 12-week treatment periods with a 4-week treatment free interval, to which 21 patients were admitted, there was no statistically significant difference between clonidine 75 microgram twice daily and placebo in the total number of headache days, migraine indices, duration of attacks, number of severe attacks and consumption of acute attack treatment. However, there was a marked reduction in number of headache days, migraine indices, duration of attacks and consumption of acute attack treatment during the second treatment period compared to the first treatment period, regardless of the treatment regime. This was presumably a result of prolonged treatment and frequent attention and not an effect of the active drug. 32 patients entered the trial, but 11 dropped out. Of the 21 patients completing the trial, 16 were women; the median age was 34 years (range 17-54 years) and the median duration of headaches 12 years (range 1-40 years). Only mild side-effects were registered and no laboratory abnormalities were seen.