It is now evident that the term Bartter syndrome does not represent a unique entity but encompasses a variety of disorders of renal electrolyte transport. Application of molecular biology techniques has permitted a better understanding of these "Bartter-like syndromes," which at present can be divided into three different genetic and clinical entities. Neonatal Bartter syndrome is observed in newborn infants and characterized by polyhydramnios, premature delivery, life-threatening episodes of fever and dehydration during the early weeks of life, growth retardation, hypercalciuria, and early-onset nephrocalcinosis. Two molecular defects have been identified: either at the gene encoding the renal bumetanide-sensitive Na-K-2Cl cotransporter (NKCC2) or the gene encoding an ATP-sensitive inwardly rectifying K channel (ROMK). "Classic" Bartter syndrome is mostly observed during infancy and childhood and is characterized clinically by polyuria and growth retardation. Nephrocalcinosis is not present. Very recently, either deletions or mutations at the gene encoding a renal chloride channel (ClC-Kb) have been identified. Gitelman syndrome is observed in older children and adults presenting with intermittent episodes of muscle weakness and tetany, hypokalemia, and hypomagnesemia. Mutations at the gene encoding the thiazide-sensitive Na-Cl cotransporter have been identified in the majority of patients studied. Obviously the validity of this classification must be confirmed in the near future when all mutations have been described and genotypic-phenotypic correlations are better defined.