ABSTRACT Patients with small-cell lung cancer and hyponatremia were examined for the syndrome of inappropriate antidiuresis (SIAD). A comparison was made between the definition based on hyponatremia, serum hypoosmolality and urine hyperosmolality (classic SIAD, 12 patients) and a definition based on measurement of plasma ADH concentration by radioimmunoassay (RIA-SIAD, nine patients) and patients without SIAD (eight patients). A standard water load test was performed as a reference before initiation of cytostatic treatment. All tests were repeated if remission of the malignant disease occurred. RIA-SIAD patients were a subgroup of classic SIAD patients, with more pronounced homeostatic abnormalities. Biochemical abnormalities were reduced after tumor regression but a completely normal renal water handling was achieved in only few patients, even when complete remission of the tumor was achieved, presumably due to the persistence of subclinical disease. However, an effect of other yet unknown factors might be of influence.