Defective jejunal brush border membrane sodium/proton exchange in association with lethal familial protracted diarrhoea. The spectrum of clinical disease associated with specific defects in jejunal brush border membrane sodium/proton exchange is poorly defined and only two patients have been described so far. Jejunal brush border membrane transport studies were performed in a boy who presented with lethal familial protracted diarrhoea in the first few days of life. Using jejunal brush border membrane vesicles prepared from conventional jejunal biopsy specimens, initial sodium uptake under H+ gradient conditions was found to be only 6% of the mean control value. In contrast, sodium stimulated glucose uptake was normal. Our data confirm the importance of a congenital defect in this exchanger as a cause of severe sodium-losing diarrhoea and extend the spectrum of disorders characterised by a specific defect in brush border membrane Na+/H+ exchange to include some forms of lethal familial protracted diarrhoea. 