In the past year, significant progress has been achieved in the research on aquaporins (AQPs), a family of structurally related molecular water channels. Three novel AQPs were identified, giving a total of ten mammalian AQPs. An important step forward in identifying the aqueous pore in AQP molecules was the determination of the three-dimensional structure of AQP1. The expression pattern of individual AQPs in different tissues was determined in more detail and AQP-knockout mice have been generated. The discovery of a severe urinary concentrating defect in AQP1-knockout mice was remarkable. Only AQP2, the vasopressin-sensitive water channel in the kidney, which is mutated in autosomal recessive and dominant cases of nephrogenic diabetes insipidus, has been shown to be involved in human disease. The finding of changed AQP2 expression in several acquired water balance disorders may pave the way toward developing treatments for these clinical problems.