Summary Renal function was assessed in 270 patients with brucellosis. In 58 consecutive patients of this total, plasma and urinary osmolality and urinary sodium excretion as well as adrenal and thyroid functions were measured prospectively. Results of these measurements satisfy criteria for the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). Such inappropriate secretion explains the asymptomatic hyponatraemia observed in our patients. The hyponatraemia together with the usual haematological findings and cellular derangement of the liver may be useful in the differential diagnosis of brucellosis which should be considered as one of the clinical conditions leading to SIADH.