Thin films of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) are prepared in which the cylindrical PMMA microdomains are oriented normal to the substrate. When the PS block is crosslinked, pores with tailored diameters ranging from 3 nm to similar to8 nm, are generated at the center of PMMA microdomains (see Figure). The distance between the pores and their lateral distribution reflects that of the initial copolymer template.