Repeated thinning to five growing-stock levels resulted in widely differing tree sizes and volumes per acre after 30 years. Largest trees but the least cubic-volume yield per acre were produced in the heaviest thinning level, whereas highest board-foot yields were found in intermediate thinning levels. partial defoliation by larch casebearer (Coleophora Iaricella Hubner), drought, and top damage from ice occurred, and site trees grew less in height than expected during the 30-year study. Curvilinear increases in periodic annual increments of both basal area and cubic volume generally occurred with increasing stand density, but increments dropped off at the highest stand densities for some periods. Anticipated patterns for these increments were found after fitting a model that included stand density index, height increments of site trees, and dummy variables for periods as independent variables. heavy thinning did not increase the age of culmination of cubic-volume means annual increment as expected. Thinning stands of larch to densities as low as 50 percent of 'normal' results in little loss of basal-area growth, a moderate loss in volume production, and a large increase in tree diameter. Thinning is necessary in many larch stands to maintain vigorous, rapidly growing trees.