The efficacy of the modified-release formulation of tizanidine (Sirdalud) was compared with placebo in a randomized, double-blind, parallel-group study of 138 women and 47 men, aged 18 to 79 years, with a history of chronic tension-type headache (IHS categories 2.2 and 2.3). The treatment period was 6 weeks preceded by a 2-week prerandomization period. The patients were randomly assigned to receive 6-mg Sirdalud, 12-mg Sirdalud MR, or placebo. The study medication was taken once per day, orally in the evening. Efficacy was measured by visual analog scale, the number of headache-free days, the daily duration of headache, and the use of paracetamol. The primary end point was the severity of daily headache derived from visual analog scale scores covering the last 2 treatment weeks. One hundred sixty patients (56 in the 6-mg group, 49 in the 12-mg group, and 55 in the placebo group) completed the study. The severity of the headache decreased similarly in the treatment groups and the placebo group. The visual analog scale values decreased from the prerandomization values by 53% in the 6-mg group, 48% in the 12-mg group, and 52% in the placebo group. The modified-release formulation of tizanidine in doses up to 12 mg taken in the evening is not superior to placebo in the treatment of chronic tension-type headache. The placebo effect was unexpectedly strong in the present study, supporting the view that psychophysiological mechanisms are of considerable importance in sustaining chronic tension-type headache.