Urinary sodium excretion before and during insulin therapy was studied in six patients with poorly controlled diabetes. One subject had ketoacidosis; manifestations in the other five ranged from slight ketoacidosis to hyperglycemia alone. All patients retained sodium when diabetes was controlled by insulin, the mean positive sodium balance being 286 mEq. over three days. The diminished urinary sodium excretion did not correlate with the fall in glucosuria or ketonuria; insulin withdrawal studies in two patients indicated that sodium depletion during insulin lack is not a prerequisite for sodium retention during retreatment Im-munoreactive glucagon fell from elevated levels as control of diabetes was established. It is concluded that sodium retention occurs regularly during insulin treatment of poorly controlled diabetes, whether ketoacidosis is present or not. The positive sodium balance should be anticipated when insulin therapy is instituted.