This double-blind, randomized, crossover study compared the efficacy and safety of danazol (100 mg twice daily) with matching placebo in the treatment of severe premenstrual syndrome. Nineteen patients were randomly allocated to receive danazol for 3 months followed by placebo, and 18 to receive treatment in the reverse order. Assessments of overall condition showed improvement to be statistically significantly more likely with danazol than with placebo (P less than 0.001) after 3 months' treatment. Furthermore, daily visual analogue scale assessments demonstrated statistically significantly better premenstrual scores with danazol in comparison to placebo for breast discomfort, irritability, depression, anxiety, mood swings, crying, depressed libido and abdominal swelling. It is concluded that danazol provides effective and generally well tolerated treatment for severe premenstrual syndrome.