An increased activity of sodium-lithium countertransport (SLC) is a common finding in patients who have essential hypertension. The evidence that a similar dysfunction is shared also by patients with type 1 diabetes and nephropathy has suggested the hypothesis that a predisposition to essential hypertension may be the factor that, along with hyperglycemia, underlies the development of diabetic nephropathy. Despite the initial enthusiasm surrounding the potential use of SLC activity as a marker for the early detection and treatment of individuals who are predisposed to hypertension and diabetic nephropathy, its use has been so far restricted to epidemiologic studies, as specificity and sensitivity of the test are still too low to justify any clinical use. The recent finding, however, that the measurement of kinetic parameters of SLC can significantly increase the power to discriminate among individuals with and without hypertension or diabetic nephropathy could be of help toward a future clinical use of the measurement of this membrane transport. A second major point relates to the possibility that SLC per se might be directly involved in the pathogenesis of essential hypertension and diabetic nephropathy. This case has never been fully tested, as the gene responsible for this membrane transport has been, until recently, unknown. The recent identification of an alternative splicing of the first isoform of Na-H exchange that mediates SLC activity should allow for a rapid comprehension of the role of this transport in the pathophysiology of essential hypertension and diabetic nephropathy.