The incidence of forest dieback in the Northern Hardwoods biome of Canada and the UnitedStates was determined for period from 1910 to 1990. Information from annual forest servicepathology inventories in the two countries and other published literature was coded to estimateyearly the severity and areal extent of dieback on white/yellow birch and sugar maple from 1910to 1990. Principal dieback episodes occurred as distinct waves coincident with maturation of theforest population in each of six regions. These episodes endured an average of 11 years. It ishypothesized that, once forest populations are mature, they are susceptible to extreme stressessuch as freezing and drought which serve to synchronize the onset and subsidence of majordieback episodes.