This open study evaluates the effectiveness and safety of topiramate for the prophylaxis of chronic tension-type headache. Fifty-one patients were enrolled, of whom 46 completed 24 weeks of treatment with topiramate. Daily dosing was titrated from 25 mg to 100 mg by treatment week 4. The primary efficacy parameter was headache frequency at weeks 13-24 compared with baseline. Headache frequency declined from 23.50 +/- 5.32 days (baseline, mean, SD) to 12.58 +/- 6.28 days at weeks 13-24 (P < 0.0001), with frequency of severe headaches dropping from 8.18 to 3.14 days (P < 0.0001). The average headache intensity dropped from 6.13 to 2.07 on the visual analogue scale (P < 0.0001). These parameters were not significantly reduced at weeks 5-12. A 50% reduction in headache frequency was achieved in 73% of patients at weeks 13-24. Also improved were mood, sleep, quality of life (all parameters, P < 0.0001) as well as the Beck Depression Inventory-II (P < 0.0001). In addition, a highly significant weight loss of 2.14 kg (mean) was observed between baseline (71.64 +/- 10.65 kg) and week 24 [69.50 +/- 10.04 kg (SD)] (P < 0.0001). There were only few side-effects, none of these rated severe. The results provide preliminary confirmation of the efficacy and tolerability of topiramate in the prophylaxis of chronic tension-type headache.