Six of 146 patients at our institution developed mediastinal emphysema following marrow transplantation. Five patients were the recipients of marrow from their HLA fully matched sibling donors and one patient had an autologous marrow transplant. Features common to all included single-dose total body irradiation and high-dose systemic adrenocorticosteroids. Five of the patients had clinical and radiological evidence of recent or active interstitial pneumonitis. The clinical implications and possible aetiologies of mediastinal emphysema developing in the post-transplant patient are discussed.