We studied the pathogenesis of cyclosporine-induced hypomagnesemia in five patients with nephrosis. Serum magnesium concentrations and urinary excretion of magnesium were reduced by the therapy. In contrast, the magnesium concentrations in mononuclear blood cells were increased. We conclude that short-term use of cyclosporine induces an intracellular shift of magnesium and causes hypomagnesemia.