Arginine vasopressin (AVP) has a key role in osmoregulation by facilitating water transport in the collecting duct. Recent evidence suggests that AVP may have additional effects on renal function and favor cyst growth in polycystic kidney disease. Whether AVP also affects kidney structure in the general population is unknown. We analyzed the association of copeptin, an established surrogate for AVP, with parameters of renal function and morphology in a multicentric population-based cohort. Participants from families of European ancestry were randomly selected in three Swiss cities. We used linear multilevel regression analysis to explore the association of copeptin with renal function parameters as well as kidney length and the presence of simple renal cysts assessed by ultrasound examination. Copeptin levels were log-transformed. The 529 women and 481 men had median copeptin levels of 3.0 and 5.2 pmol/L, respectively (P,0.001). In multivariable analyses, the copeptin level was associated inversely with eGFR (b=22.1; 95% confidence interval [95% CI], 23.3 to 20.8; P=0.002) and kidney length (b=21.2; 95% CI, 21.9 to 20.4; P=0.003) but positively with 24-hour urinary albumin excretion (b=0.11; 95% CI, 0.01 to 0.20; P=0.03) and urine osmolality (b=0.08; 95% CI, 0.05 to 0.10; P,0.001). A positive association was found between the copeptin level and the presence of renal cysts (odds ratio, 1.6; 95% CI, 1.1 to 2.4; P=0.02). These results suggest that AVP has a pleiotropic role in renal function and may favor the development of simple renal cysts. The antidiuretic hormone arginine vasopressin (AVP) plays an essential role in osmoregulation by facilitating water transport in the collecting duct. 1 This effect is mediated by the stimulation of the G protein-coupled vasopressin V2 receptor (V2R), increasing cAMP levels and leading to a transient increase in osmotic water transport across the principal cells. 2 Furthermore, it was recently demonstrated that V2Rs are also expressed in the thick ascending limb of the human kidney. 3,4 In addition to the V2R, AVP acts on the V1a receptor (Ca 2+ as second messenger), which is distributed in the endothelial cells lining systemic and renal blood vessels and capillaries (including vasa recta), as well as the glomerulus, the macula densa, the medullary interstitial cells, and the cortical collecting duct. [5][6][7] Of note, V1a receptors seem to