Under double-blind, crossover conditions, 43 women with primary dysmenorrhea received ketoprofen, ibuprofen, and placebo during three consecutive menstrual cycles. Pain intensity and pain relief were determined before and for 6 hours after the loading dose (ketoprofen 150 mg, ibuprofen 800 mg) and before and 2 hours after the maintenance dose (ketoprofen 75 mg, ibuprofen 400 mg). Mean pain intensity difference and pain relief scores consistently indicated greater pain relief after the loading doses of ketoprofen and ibuprofen than after placebo. Significant (P less than 0.05) mean changes that were measured by 13 indices of analgesia after the loading doses of both ketoprofen and ibuprofen indicated greater efficacy for the active treatments than for placebo. The patients' global evaluations after the loading doses were significantly (P less than 0.05) better for the active treatments than for placebo. The efficacy results were similar after the maintenance doses. The rates of a "good" to "excellent" response were 77% for ketoprofen, 73% for ibuprofen, and 35% for placebo. Ketoprofen and ibuprofen were equally well tolerated, the most frequent adverse experiences being gastrointestinal symptoms for ketoprofen and central nervous system side effects for ibuprofen.