The vasopressin inhibitor tolvaptan is clinically effective in slowing growth of renal cysts and reduction in estimated glomerular filtration rate (eGFR) in autosomal dominant polycystic kidney disease (ADPKD), but these effects are mitigated by the associated polyuria. Changes of total kidney volume, eGFR, and symptoms will guide physicians and patients in tolvaptan treatment. Guidance about when to initiate treatment in the course of ADPKD may be forthcoming. Ongoing long-term observations will inform future recommendations about tolvaptan use in ADPKD.