Hydrologically sensitive tree-ring chronologies from Pinus flexilis in California and Alberta were used to produce an AD 993--1996 reconstruction of the Pacific Decadal Oscillation (PDO) and to assess long-term variability in the PDO's strength and periodicity. The reconstruction indicates that a ~50 to 70 year periodicity in the PDO is typical for the past 200 years but, was only intermittently a strong mode of variability prior to that. Between AD 1600 and 1800 there is a general absence of significant variability within the 50 to 100 year frequency range. Significant variability within in the frequency range of 50 to 100 years reemerges between AD 1500 and 1300 and AD 1200 to 1000. A prolonged period of strongly negative PDO values between AD 993 and 1300 is contemporaneous with a severe medieval megadrought that is apparent in many proxy hydrologic records for the western United States and Canada.