This paper is a summary of results obtained in our studies on leptinemia in patients with chronic renal failure treated with recombinant human erythropoietin (rHuEPO), in kidney transplant patients, in patients with essential hypertension, and in pregnant women with preeclampsia. In this study, we found that rHuEPO treatment has a suppressive effect on leptinemia in patients with endstage renal failure. These results suggest that the appetite stimulating effect of rHuEPO may be mediated by a reduction of leptin synthesis and release. At the early stage of successful kidney transplantation, a significant decline of leptinemia was noticed, which was not related either to the excretory function of the graft or the kind and dose of immunosuppressants. In kidney transplant patients with grafts functioning well for 2.5 years, significantly elevated leptinemia was found. From these results, we may conclude that factors other than the excretory function of the graft and the kind and dosage of immunosuppressants may be involved in the pathogenesis of abnormal leptinemia in these patients. Both in normotensive subjects and patients with essential hypertension, a positive correlation was found between leptinemia and mean blood pressure, suggesting that leptin may be involved in the regulation of blood pressure. Both healthy and preeclamptic pregnant women show higher leptinemia than nonpregnant women. In preeclamptic women, leptin levels in maternal vein blood, umbilical cord blood, and amniotic fluid were significantly higher than respective values found in healthy pregnant women. In contrast to healthy pregnant and nonpregnant women, in women with preeclampsia, no correlation was found between the body mass index (BMI) and leptinemia. In preeclamptic women the abnormally elevated leptinemia was not related to blood pressure. Finally, no correlation was found between leptinemia in maternal and umbilical cord blood. From these studies, it follows that the elucidation of abnormal leptin secretion in the pathogenesis of preeclampsia needs further study.