A survey was made of 10,506 reports to the WHO Collaboration Centre for International Drug Monitoring from five countries concerning headache, migraine, aggravated migraine and intracranial hypertension associated with drugs. The ten drugs most frequently reported to be associated with headache were indomethacin, nifedipine, cimetidine, atenolol, trimethoprim-sulphamethoxazole, zimeldine, glyceryl trinitrate, isosorbide dinitrate, zomepirac and ranitidine. Regarding migraine, oral contraceptives were also among the most implicated drugs. Most reports of intracranial hypertension concerned tetracyclines, isotretinoin and trimethoprim-sulphamethoxazole. Vasodilatation and salt and water retention with subsequent redistribution of intracranial fluid seem to be common mechanisms underlying drug-related headache. For certain frequently reported drugs, however, the mechanisms of the headache are unknown.