FOLLOWING the discovery(1) of electroluminescence in poly(p-phenylenevinylene) (PPV), considerable effort has been directed towards the realization of optoelectronic devices based on semiconducting conjugated polymers of this type(2-6). But the viability of these materials for such applications depends critically on the nature of the photoexcited states-in particular, whether they are predominantly non-emitting interchain species(7-9) or emitting intrachain species(10), One way to study this fundamental issue is in a device structure known as a microcavity(11), which offers the possibility of using quantum electrodynamic effects to alter (and hence probe the nature of) spontaneous and stimulated emission in these materials(12-17). Here we make use of such a structure to demonstrate optically driven laser activity in devices based on solid films of PPV. This demonstration of lasing provides direct support for a model(10) in which the main photoexcitation in PPV is an emissive intrachain species, and opens the possibility of electrically driven polymer-based lasers.