Lung allograft rejection can usually be diagnosed by the appearance of infiltrates on plain chest roentgenograms when these are interpreted in the light of other clinical and bacteriologic information. Large pulsed intravenous doses of methylprednisolone were usually effective in reversing lung allograft rejection that occurred in immunosuppressed dogs. In 10 of 15 animals the presence of moderate to severe rejection and its effective reversal with treatment were documented with roentgenograms and histologic sections. This ability to reverse the manifestations of lung allograft rejection, when they occur, has helped in the management of human lung allograft recipients.