Large woody debris (LWD) in the form of standing dead trees and down logs is an important structural and functional component of forest ecosystems and is a key factor linking riparian forests and streams. We examined the influence of forest seral stage (old-growth vs. second-growth forest) on terrestrial (snags and down logs) and in-stream LWD abundance in a riparian forest-stream system in Cook Forest State Park, Pennsylvania. Snag density, but not basal area, was significantly greater in second-growth forest due to elevated mortality of eastern white pine (Pinus strobus L.). Abundance of both terrestrial and in-stream LWD was significantly greater in the old-growth forest-stream system than in second-growth. Eastern hemlock, Tsuga canadensis (L.) Carr., strongly dominated the volume of both terrestrial and in-stream LWD in the old-growth stand and adjacent stream reaches. Terrestrial and in-stream LWD loadings that we recorded are similar to loadings occurring in forest-stream systems elsewhere in the eastern United States, especially the Appalachian region, suggesting that these values may be useful targets for LWD enhancement projects for Allegheny High Plateau forest-stream systems.