Whole blood 5HT levels were measured in seven female migraine sufferers with chronic daily headache due to medication abuse, before and after abrupt medication withdrawal. A statistically significant increase in 5HT levels, from mean 4.89 mumol/l to mean 6.59 mumol/l (p < 0.05, Wilcoxon signed rank test), occurred after 4 weeks of withdrawal. We conclude from this pilot study that 5HT may be important in the physiopathogenesis of chronic daily headache. Alternatively, reduced 5HT may be the result of chronic daily headache or else an epiphenomenon.