Thirty-five patients (16--23 years old) who had severe primary dysmenorrhea were each treated with 500 mg of mefenamic acid every eight hours for a maximum of three days during menstruation for three consecutive cycles. A total of 194 treated cycles could be evaluated, 110 cycles with mefenamic acid and 84 with placebo. Mefenamic acid produced complete relief of all the symptoms of dysmenorrhea in 31 (88.6%) patients in all 98 treated cycles and, in another two patients, moderate relief in five of the six cycles. While on placebo, only five patients (13%) experienced moderate to slight relief in 11 of the 15 cycles. It is concluded that the mefenamic acid is safe and effective in most patients for the relief of primary dysmenorrhea and represents a rational short-term therapy for this syndrome.