The serum sodium concentration (SNa + ) was followed-up in 200 patients after craniocerebral trauma associated with diabetes insipidus and significant loss of solute-free water. Severe hypernatraemia (SNa + > 160 mmol/l) was observed in 26% of cases and excessive (SNa + > 170 mmol/l) in 6%. For approximate calculation of water deficit (WD) the formula recommended in textbooks does not take into account changes in body weight (BW) caused by loss of solute-free water. In the submitted communication a formula is derived taking into account current BW which is significantly lower than that before dehydration (and which is usually unknown). The recommended formula is as follows.In patients with severe hypernatraemia calculation of WD respecting and not respecting the above correction causes differences of about 15%.

        Key words: hypernatraemia, calculation of water deficit.