At 4 weeks of age, an infant with Down syndrome developed severe dehydration and salt loss with the typical features of pseudohypoaldosteronism (PHA). Plasma renin activity, 11-deoxycorticosterone, corticosterone and aldosterone levels were all increased several-fold over the normal range for age, thus excluding an adrenal biosynthetic defect. Clinical condition, hyponatraemia and hyperkalaemia could be rapidly normalised by the ion exchange resin Resonium A administered first as enema and later orally (3 g/day). At that time, no further salt supplementation was necessary. At 18 months of age, Resonium A could be completely withdrawn with neither clinical deterioration nor electrolyte abnormalities. However at 3 1/2 years of age, plasma renin activity and aldosterone were still markedly elevated while precursor steroids were normal and the clinical condition satisfactory. No side effects were observed with the Resonium A therapy. The combination of trisomy 21 and PHA is very unusual. Similarly, the successful treatment of severe renal salt loss during infancy by sodium supplementation and concomitant potassium withdrawal via an oral ion exchange resin has not yet been described and warrants further therapeutic trials.