Western spruce budworm defoliated area in the Northern Region has differed significantly across three discrete geographic zones during the past decade. Aerially visible defoliation in northern Idaho increased from 1.7 million acres in 1969 to a high of 2.2 million acres in 1974, and declined to none in 1979. Defoliated area in western Montana increased from 1.8 million acres in 1969 to a high of 2.8 million acres in 1972 and declined to 0.6 million acres in 1979. Conversely, defoliated area in eastern Montana fluctuated at low levels between 0.1 and 0.7 million acres between 1969 and 1974, and then rose to a high of 1.6 million acres in 1979. Analysis of defoliation trend, the ratio of acres defoliated in the current year by acres defoliated the prior year, and weather during budworm larval and pupal periods during the past decade revealed the following relationships: Defoliation trend in all three geographic areas varied (a) directly with mean maximum temperature during May, June, and July of the year before, and (b) inversely with frequency of measurable precipitation during May, June, and July of the year before. Based on warm, dry conditions throughout the Region in 1979, we predict a general increase in budworm populations in the Northern Region in 1980.