Hyponatremia is the one of the most common electrolyte abnormality in the clinical practice and is associated with increased morbidity and mortality. Decreased serum sodium levels are occasionally observed in patients with diabetes mellitus, especially in those, who pre-sent with the diabetic ketoacidosis. It can develop at the each stage of a treatment, as a complication of hyperglycemia and intensity of the therapy, but also the other underlying causes should be consider. In this report we present a patient with symptomatic hyponatremia in the new diagnosed patient with type 1 diabetes mellitus.