Liddle's syndrome is an autosomal dominant form of salt-sensitive hypertension and has been shown to be caused by missense or frameshift mutations in the amiloride-sensitive epithelial sodium channel (ENaC), which is composed of three subunits: alpha, beta, and gamma. All disease mutations either remove or alter amino acids of the target proline-rich PPPxY sequence (PY motif) of beta- or gamma-ENaC and result in increased channel activity. In this report, we present a family with Liddle's syndrome whose abnormality is caused by a novel missense mutation, P616R, in the PY motif of the betaENaC. Functional studies using the P616R mutant expressed in Xenopus oocytes showed an approximately 6-fold increase in the amiloride-sensitive sodium channel activity compared with that of the wild type. These findings provide additional clinical evidence that a conserved PY motif is critically important for the regulation of ENaC activity.