Additional Study Information: This study is for patients with autosomal dominant polycystic kidney disease (ADPKD). The purpose of this study is to test the effectiveness of a study drug called tolvaptan. Tolvaptan continues to be studied in this trial as a possible treatment for ADPKD. ADPKD is a disease that causes kidney cysts (cyst are like fluid-filled balloons), worsening kidney function, blood in the urine, kidney pain, high blood pressure, kidney stones, kidney infections, and cysts in the liver or other parts of the body and sometimes heart or blood vessel abnormalities. For people with ADPKD, the kidneys respond abnormally to the hormone vasopressin that may be involved in cyst development or growth in humans. Tolvaptan interferes with the effect of vasopressin on the kidney, and reduces cyst growth when given to animal models of ADPKD. Its effects will continue to be studied in humans to further test how useful tolvaptan will be intreating ADPKD.