Dexketoprofen, the pure S(+)-enantiomer of ketoprofen, is a promising new analgesic, but few clinical trials have yet examined its efficacy and tolerability. In this study, patients with a history of primary dysmenorrhea were treated with dexketoprofen doses of 12.5 and 25 mg, ketoprofen 50 mg, and placebo using a randomized, four-way crossover design. Efficacy analyses showed that dexketoprofen 12.5 and 25 mg and racemic ketoprofen 50 mg significantly reduced pain intensity compared with placebo from 1 h after dose to 4-6 h after dose. Interestingly, dexketoprofen at 12.5 mg was significantly superior to placebo at 30 min after dose. Mean pain relief scores also demonstrated that both doses of dexketoprofen and racemic ketoprofen were significantly superior to placebo at 1-6 h after the first dose. No indices of analgesic efficacy showed any significant differences between the two doses of dexketoprofen or between dexketoprofen and ketoprofen. After repeated dose administration, similar results were obtained. There were no significant effects of any treatment on activities of daily living, menstrual flow, or associated symptoms. Dexketoprofen was effective, well tolerated, and had no difference in the incidence of adverse events compared to ketoprofen or placebo.