Inclusion Criteria:

• Subjects who have successfully completed a Phase 1, 2, or 3 tolvaptan ADPKD or renal impairment trial, with a confirmed diagnosis of ADPKD

Exclusion Criteria:

• Subjects unable to provide written informed consent
• Subjects (men or women) who will not adhere to the reproductive precautions as outlined in the Informed Consent Form
• Subjects (women only) with a positive urine pregnancy test
• Subjects who are pregnant or breast-feeding
• Subjects unable to take oral medications
• Subjects who have allergic reactions to tolvaptan or chemically related structures such as benzazepines (benzazepril, conivaptan, fenoldopam mesylate or mirtazapine)
• Subjects who have disorders in thirst recognition or an inability to access fluids
• Subjects with critical electrolyte imbalances, as determined by the investigator
• Subjects with or at risk of significant hypovolemia, as determined by investigator
• Subjects with anemia, as determined by investigator
• Subjects with a history of substance abuse (within the last 3 years)
• Subjects taking other experimental (ie, non-marketed) therapies or current participation in another clinical drug or device trial; current participation in the off-drug follow-up period of another ADPKD trial with tolvaptan is permitted

Efficacy Analysis Exclusion Criteria:

• Subjects unable to complete MRI assessments(eg, subjects with ferro-magnetic prostheses, aneurysm clips, severe claustrophobia)
• Subjects who have taken a vasopressin antagonist (outside of previous participation in a tolvaptan trial)
• Subjects unable to comply with anti-hypertensive or other important medical therapy
• Subjects with advanced diabetes
• Subjects taking medications or having an illness that could confound endpoint assessments (including taking approved therapies for the purpose of affecting PKD cysts)