There are different causes of hyponatremia, but the clinical course is mostly uniform, especially in sucklings: vomiting, dehydration and failure to thrive. The simple basic examination, which enables the application of modern rational diagnostic algorithms, is the evaluation of the state of dehydration, the determination of sodium and potassium levels in blood serum and sodium concentration in urine (before the initiation of therapy, if possible). Various causes of severe hyponatremia resulting from absolute or relative deficit of aldosterone, associated with clinical signs of serious illness, are demonstrated on the cases of three children at the age of two to six weeks.

        Key words: failure to thrive, hyponatremia, hypoaldosteronism, pseudohypoaldosteronism