The accurate diagnosis of lung transplant rejection requires histologic examination of the grafted lung. Because transbronchial lung biopsy has been advocated as an effective diagnostic procedure for a variety of lung diseases, it was elected to assess this technique in rejecting transplanted lungs. Twenty-two dogs received allografts and underwent simultaneous lung biopsy by 3 techniques when signs of rejection occurred. Open lung biopsy was diagnostic of rejection in all instances. Transthoracic needle biopsy correlated with the open biopsy in 59 per cent of the cases. No specimen obtained by transbronchial lung biopsy provided sufficient material to permit fulfillment of the strict histologic criteria needed to diagnose allograft rejection. Although transbronchial lung biopsy is successful in many pulmonary infiltrative processes, it appears to be inadequate for the diagnosis of lung allograft rejection.