Danazol, approved 5 years ago for the treatment of pelvic endometriosis, has recently been approved for treating cystic disease of the breast. As the mechanisms of action of this drug are made clear, the clinical syndromes that can be treated by this impeded androgen have increased dramatically. Results of controlled prospective clinical studies on the drug have increased our understanding of the pathophysiology of several poorly understood diseases. Danazol is well tolerated by most patients; the side effects are mild and reversible.