An increase in land dominated by young second-growth Douglas-fir forests in the Pacific Northwest has coincided with heightened concerns over loss of old-growth habitat. In search of options for managing young forests to provide late-successional forest structures, the Young Stand Thinning and Diversity Study was designed to test the effectiveness of modified thinning in acceleration of late-successional structural characteristics. Thinning treatments included: a control, a light thin (typical of standard commercial thins), a heavy thin (densities lower than typically prescribed), and a light thin with gaps (stands thinned lightly with the addition of 0.2 hectare patch cuts evenly spaced throughout the stand). Early response (maximum of 5–7 years post-treatment) of overstory vegetation was examined. Average growth of Douglas-fir increased in all thinned stands, but growth of the largest Douglas-fir trees was only accelerated in the heavy thin. After thinning, the canopy of all thinned treatments was initially more open than the control, but after 5–7 years the light thin was no longer significantly different from the control. The light with gaps thin had the highest variation in overstory canopy cover. Differentiation of vertical canopy structure among treatments was not evident. There was no difference in mortality among any of the treatments for most species tested; those that did had highest mortality in the control. Our results indicate that thinning can be effective in hastening development of some, but not all late-successional attributes, but such acceleration is not equivalent among the different thinning treatments.