Pendred syndrome is a recessive autosomal disorder characterized by thyroid goiter and sensorineural hearing loss. The Pendred syndrome gene (SLC26A4) encodes a new anion exchanger named pendrin which mediates iodide transport by thyrocytes and regulates ion and fluid transport by the endolymphatic sac epithelium. Pendrin defects result in inner ear malformations, with enlargement of the endolymphatic sac and duct in association with a large vestibular aqueduct. Furthermore, patients may develop endolymphatic hydrops requiring diuretic therapy, mainly in the form of thiazides. Pendrin could also account for apical Cl (-)/HCO3 (-) exchange at level of intercalated cells of the cortical collecting duct in the kidneys, however, humans with Pendred syndrome have no symptoms attributable to renal pendrin abnormalities in basal conditions. We report the case of a child with Pendred syndrome and intercurrent endolymphatic hydrops, who developed profound hypokalemia and severe hypochloremic metabolic alkalosis (potassium 1.7, chloride 70, sodium 129, HCO3 43.8, base excess+ 17.8 mmol/l, pH 7.52) following thiazide therapy. In subjects with Pendred syndrome thiazide therapy seems to provoke more severe Cl (-) and extracellular volume depletion. A possible explanation could be the defective action of the disrupted pendrin, which exacerbates the effects of the inhibition of C1 (-) reabsorption mediated by the thiazide-sensitive NaCl cotransporter (SLC12A3).