Dialysis may induce severe headache as a result of a large amount of water and electrolyte shifts. It is important to recognize it because it can be a great problem to the patient and changing dialysis parameters or methods can prevent it. In this study we investigated the frequency and clinical characteristics of headaches occurring during haemodialysis (HD). Thirty female and 33 male patients with chronic renal failure on regular dialysis for at least 6 months in the HD unit of the Internal Medicine Department from 1996 to 2000 participated in the study. The dialysis solution contained acetate in 35 patients and bicarbonate in 28 patients. In all patients capillary dialysers and Cuprophan membranes were used and every session of dialysis lasted 4 h. All patients received the same questionnaire and they were visited randomly. Dialysis headache (DH) diagnosis was made according to the criteria of the International Headache Society. Patients with primary headache and under drug treatment during HD, which can cause headache, were excluded from the study. The frequency of DH, its relation to gender, age, dialysis technique and parameters and its features were investigated. DH was detected in 48% (n=30) of the study group. Compared with dialysis solutions, no difference was found between patients with and without DH. The difference in the pre- and post-dialysis value of urea in patients with DH was statistically significant (P<0.05). Patients with DH showed significantly higher mean systolic and diastolic blood pressure predialysis values in comparison with patients without DH (systolic, P<0.001; diastolic, P<0.01), whereas post-treatment values did not differ between the two groups. Fronto-temporal location, moderate severity, throbbing quality and short duration (<4 h) were the most prevalent features of DH in patients.