Treatment for chronic tension-type headache (CTTH) is unsatisfactory. Our aim was to investigate the efficacy of the N-methyl D-aspartate (NMDA) antagonist memantine in the prophylactic treatment of CTTH. We included 40 patients in a randomized, double-blind, placebo-controlled, crossover trial. Memantine 20-40 mg/day or placebo was each given for 10 weeks separated by a 2-week wash-out period; 29 patients completed the study. The primary efficacy variable, area-under-the-headache curve (duration x intensity), did not differ between memantine (1352 +/- 927) and placebo (1449 +/- 976; P = 0.10). Headache intensity in both sexes was significantly lower on a 0-10 verbal rating scale with memantine (3.8) than with placebo (4.1; P = 0.03). In women, area-under-the-headache curve was significantly lower with memantine (1343 +/- 919) than with placebo (1555 +/- 1019; P = 0.01). The most common side-effects were dizziness and nausea. In conclusion, although no statistically significant effect was seen in the primary end-point, some beneficial effects of memantine were observed in women. Memantine was shown to reduce pain intensity in CTTH patients, albeit to a limited extent. Future NMDA antagonists with higher efficacy could be of major interest as regards the pathophysiology and future treatment of CTTH and other chronic pain disorders.