The dimensional stability of ethylene copolymers in rotational molding was studied by comparing the warpage observed for a series of conventional and single-site catalyzed ethylene copolymers. Bench-scale molding trials were carried out under controlled molding conditions. The rapid cooling of the mold using a water spray resulted in greater warpage. Under such conditions, molded parts made using the single-site resins showed less warpage compared to the Ziegler-catalyzed copolymers with otherwise comparable densities. The Ziegler-catalyzed copolymers were characterized by a faster crystallization rate, and were shown to generate larger crystallinity gradients through the part thickness during the cooling process. Second to temperature gradients, crystallinity gradients are a leading cause for the development of residual stresses and causing warpage. Differences in the crystallization rates between single-site and Ziegler-catalyzed copolymers are discussed based on their intermolecular comonomer distributions. POLYM. ENG. SCI., 49:1400-1409, 2009. (C) 2009 Society of Plastics Engineers