The ovaries and the adrenal glands contribute to the daily production of 300 micrograms of testosterone in healthy, menstruating women. The physiologic role of testosterone in women, however, remains poorly understood. Previous studies of testosterone supplementation, largely in surgically or naturally menopausal women, have reported improvements in subjective measures of sexual function, sense of well being, and variable changes in markers of bone formation and resorption. However, many of these previous studies used supraphysiologic doses of testosterone, and insensitive assays for the measurement of total and free testosterone levels that lacked precision and accuracy in the low range prevalent in women. The effects of testosterone in women on body composition, muscle performance and physical function have not been studied. Therefore, the clinical significance of androgen deficiency in women remains unclear. Thus, we do not know whether physiologic testosterone replacement of women with androgen deficiency can produce clinically meaningful improvements in sexual and cognitive functions, fat-free mass, and muscle performance, without virilizing side effects.