Mutations in the Na–K–2CI cotransporter (NKCC2), a mediator of renal salt reabsorption, cause Barrier's syndrome, featuring salt wasting, hypokalaemic alkalosis, hypercalciuria and low blood pressure. NKCC2 mutations can be excluded in some Bartter's kindreds, prompting examination of regulators of cotransporter activity. One regulator is believed to be ROMK, an ATP–sensitive K⁺ channel that ‘recycles’ reabsorbed K⁺ back to the tubule lumen. Examination of the ROMK gene reveals mutations that co–segregate with the disease and disrupt ROMK function in four Bartter's kindreds. Our findings establish the genetic heterogeneity of Bartter's syndrome, and demonstrate the physiologic role of ROMK in vivo.