The vasopressin V2 receptor antagonist (V2RA) tolvaptan is the first drug that has been shown to slow the rate of kidney function decline in patients with autosomal dominant polycystic kidney disease (ADPKD). However, V2RAs also cause polyuria, with urine output that averages 6L/d. We assessed determinants of urine volume in patients with ADPKD using V2RAs because such information may help develop strategies to improve V2RA tolerability.