The use of mefenamic acid in the treatment of premenstrual syndrome (PMS) was investigated in 15 women over six menstrual cycles. A randomized, double-blind, cross-over, placebo-controlled design was used to overcome the methodologic criticisms of other medication trials in this condition. Mefenamic acid significantly improved many of the physical, mood, and performance symptoms associated with PMS. The physical symptoms that showed marked improvement were fatigue, headache, and general aches and pains (P less than .001). Most mood symptoms were improved, the most significant being freedom from mood swings (P less than .005).