A defective luteal phase (DLP) results from a relative deficiency in secretion of progesterone by the corpus luteum. Approximately 30% of normal women show on biopsy an occasional DLP, while only 7-14% of infertile patients have a recurrent DLP. Endometrial biopsy remains the classical way to diagnose an inadequate luteal phase. However because of the discomfort and expense associated with endometrial biopsy, attention has turned to direct measurements of plasma progesterone levels as a means to rule out an inadequate luteal phase. A single plasma progesterone level of 10 ng/ml in the luteal phase or a mean concentration of plasma progesterone of 12-13 ng/ml over 4 days at mid-luteal phase seems to exclude an inadequacy of corpus luteum function. Changes in the precise pattern of pulsatile gonadotrophin secretion (due to weight, hyperandrogenism or hyper-prolactinaemia) result in derangements of ovulatory function. For the 'essential' defective luteal phase there are as yet no controlled studies that establish either its role in infertility or the value of therapy.