BACKGROUND: Hyponatremia is a frequent condition in elderly patients. In diagnostic workup, a 24-hour urine sample is used to measure urinary osmolality and urinary sodium concentration necessary to confirm the diagnosis of the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). This study was undertaken to test the hypothesis that a spot urine sample would be sufficient for urinalysis. METHODS: In nine patients with SIADH, morning spot and 24-hour urine samples were examined for osmolality and sodium concentration. Levels of arginine vasopressin, atrial natriuretic and brain natriuretic peptides, renin, and aldosterone were measured in the supine and upright positions of patients and compared with nine healthy age-matched control patients. RESULTS: The patients had low plasma osmolality (median 266 mOsm/kg) and measurable levels of arginine vasopressin (median 1.8 pg/mL). Values of osmolality in the spot urine (median 298 mOsm/kg) and in the 24-hour urine (median 215 mOsm/kg) did not differ significantly; neither did sodium concentration (medians 80 mmol/L in the spot urine versus 45 mmol/L in the 24-hour urine). Patients had significantly elevated plasma levels of brain natriuretic peptide (P = 0.007), elevated mean arterial blood pressure (P = 0.03), and lower plasma levels of creatinine (P = 0.002) compared to the controls. CONCLUSION: A spot urine sample seems to be sufficient to confirm the diagnosis of SIADH.