This study was intended to correlate different clinical and biologic parameters to better define luteal insufficiency (LI) and to contribute to a better understanding of its origin. Endometrial patterns were used as the basis for classification of clinical cases. Of 328 outpatients with menstrual disorders and/or infertility, 88 were considered to have LI. Their cycles were compared with 79 normal cycles. Two different principal endometrial patterns of LI are described: pure LI, when the endometrium is more than 2 days out of phase; and LI with persistent estrogenic influence, when the histologic estrogenic stigmata are excessive during the luteal phase. Basal body temperature charts demonstrated menstrual cycle disturbances: either ovulation delay or a slow increase in temperature (longer than 2 days). Plasma steroid concentrations also demonstrated a perturbation of the entire menstrual cycle: progesterone levels were statistically significantly lower in LI than in normal cycles and this defect was worse when the estrogenic influence was persistent; the preovulatory estradiol peak was disturbed in all circumstances, as was the concentration of endometrial steroid receptors. These simultaneous abnormalities strongly suggest a central origin of LI.