Objective of the research: to study kidneys physiological state and development of functional diagnostics method of the syndrome of inadequate secretion of antidiuretic hormone (SIADH). Materials and methods: 28 patients with acute pneumonia were examined, including 9 with SIADH and the comparison group of 11 children with mild bronchial asthma in the remission stage at a routine examination. The osmolality of blood serum and urine was measured using osmometry, the concentration of sodium, potassium, chlorine ions by flame photometry, ion-selective electrodes, the concentration of vasopressin, copeptin using ELISA. Results: hypoosmolality and decrease of Na ion concentration was detected in the serum of children with SIADH. The water load in the volume of 10 ml/kg of body weight in the comparison group and in patients with pneumonia increased renal excretion of solute free water, with SIADH increased its reabsorption In the initial period, the children of the studied groups had a close correlation of the osmotic-free and Na-free water clearance, the test with 1% water load revealed a qualitative difference – a decrease in clearance with SIADH and an increase with pneumonia. In patients with pneumonia after a water load the concentration of vasopressin and copeptin in the urine is reduced, with SIADH this does not occur. The correlation of vasopressin and copeptin excretion under water load in children with SIADH was established.