The growth of the mammary glands in experimental animals has been extensively investigated and has been found to depend primarily on the estrogenic hormones.1 Complete development in most mammals is dependent on the simultaneous action of estrogenic and corpus luteum hormones.2 In spite of the excellent experimental background, I have been unable to find records of carefully controlled studies of human breast growth. I therefore performed studies on three women lacking mammary development and exhibiting signs of marked hypogonadism. In the first part of the study the hormones were administered by subcutaneous injection. I was able to demonstrate that, by the injection of from 150,000 to 350,000 international units of estrone or of estradiol benzoate per week, active mammary growth could be produced in patients who previously had no visible or palpable breast tissue. The results were recorded by frequent caliper measurements and photographs. During the treatment vaginal