We have described a cyclic steroid replacement regimen of oestradiol and progesterone which is able to produce physiological concentrations of these steroids in plasma indicative of a normal menstrual cycle. The stimulation of a secretory endometrium at the appropriate time in this artificial menstrual cycle bears testament to the suitability of the treatment and has led to the establishment of pregnancy in three women with complete ovarian failure or ovarian agenesis when used in combination with the established techniques of in vitro fertilization and embryo transfer. The steroid replacement regimen can easily be adjusted to maintain pregnancy until the time of luteoplacental shift, based on the close monitoring of the plasma steroid and hCG levels in the pregnant individual. The implications of this form of treatment for other disease states and for our understanding of the mechanisms involved in ovarian hormone action, pregnancy maintenance and parturition are immense.