Hyponatraemia is one of the most frequent disorders of the electrolyte metabolism. One of the causes may be also the syndrome of inadequate antidiuretic hormone secretion. The cause of increased production of antidiuretic hormone can be not only a pathological condition affecting the hypothalamo-pituitary system but also ectopic production of antidiuretic hormone or peptide with the same effect on the distal portion of the nephron. A classical type of malignant tumour where the syndrome of inadequate antidiuretic hormone secretion develops is small cell lung cancer. It is also known that some pharmaceutical preparations stimulate the output of antidiuretic hormone or potentiate its effect.

        Key words: Hyponatraemia - SIADH - Paraneoplastic ADH secretion - Small cell lung cancer