The effect of roughness on the dewetting behavior of polyethylene thin films on silicon dioxide substrates is presented. Smooth and rough silicon dioxide substrates of 0.3 and 3.2-3.9 mn root-mean-square roughness were prepared by thermal oxidation of silicon wafers and plasma-enhanced chemical vapor deposition on silicon wafers, respectively. Polymer thin films of similar to 80 nm thickness were deposited by spin-coating on these substrates. Subsequent dewetting and crystallization of the polyethylene were observed by hot-stage optical microscopy in reflection mode. During heating, the polymer films melt and dewet on both substrates. Further observations after cooling indicate that, whereas complete dewetting occurs on the smooth substrate surface, partial dewetting occurs for the polymer film on the rough surface. The average thickness of the residual film on the rough surface was determined by ellipsometry to be a few nanometers, and the spatial distribution of the polymer in the cavities of the rough surface could be obtained by X-ray reflectometry. The residual film originates from the impregnation of the porous surface by the polymer fluid, leading to the observed partial dewetting behavior. This new type of partial dewetting should have important practical consequences, as most real surfaces exhibit significant roughness.