The present invention relates to phase separated polymeric regions and to their use in conjunction with implantable or insertable medical devices. In some aspects of the invention, phase separated polymeric regions are provided that include (a) at least one biostable polymeric phase and (b) at least one biodisintegrable polymeric phase, which is of nanoscale dimensions and which undergoes biodisintegration such that the phase separated polymeric region becomes a nanoporous polymeric region in vivo. Other aspects of the invention are directed to methods of making implantable or insertable medical devices having at least one nanoporous polymeric region. These methods include (a) providing a phase separated polymeric region comprising a stable polymeric phase and a disintegrable polymeric phase of nanoscale dimensions, (b) selectively removing the disintegrable polymeric phase thereby producing the nanoporous polymeric region. In still other aspects, implantable or insertable medical devices are provided which have phase separated polymeric regions that include (a) at least one block copolymer having at least one biostable polymer block and at least one biodisintegrable polymer block and (b) at least one therapeutic agent which is released in vivo upon implantation or insertion of the medical device.