We have performed a longitudinal study of the hypothalamic-pituitary-ovary axis and adrenal function during the menstrual cycle in 26 women with inactive or quiescent systemic lupus erythematosus (SLE) and in 21 healthy women as controls. In all women studied, the menstrual cycle was proved to be ovulatory by ultrasound imaging and detection of an appropriately elevated value of peak serum progesterone. Blood samples were obtained for hormonal assay on several days of the menstrual cycle studied. Patients with SLE had lower peak and day-7 postovulation serum progesterone concentrations which were both significantly lower than those found in controls, whereas no major differences were observed for any of the other hormones analysed. Although the significance of the inadequate production of progesterone we have observed in SLE women with inactive disease is not clear, our data seem to indicate that a relative deficiency of progesterone might be considered among the potential predisposing factors for development of SLE in women.