Prophylactic cranial irradiation (PCI) has been an accepted part of the management of both limited and small-cell lung cancer; however, the data that support its use in limited-stage disease is based on an analysis of trials done before currently accepted approaches to staging (i.e., brain MRI and/or PET scanning) were available. For extensive disease, data are available from two randomized studies that are in direct conflict. This article explores the basic rationale for PCI and the evidence indicating that it is time to readdress the question of its routine use.