Lymphatic obstruction has not been emphasized as a feature of lung allograft rejection. However, accumulation of fluid and cellular infiltrate, aggravated by lymph stasis, results in impaired lung function. In this study, lung specimens were recovered at varying times up to 133 days after either reimplantation (7 dogs) or allografting (29 dogs). Azathioprine and prednisone were administered to 17 allograft recipients. The presence of abnormally dilated perivascular, peribronchiolar, and subpleural lymphatic channels was a consistent histological finding, most striking in specimens recovered from untreated allograft recipient dogs. Attenuated lymphatic alterations were noted in immunosuppressed allograft recipients. In these animals the pulmonary lymphatics seemed to be ineffectual in clearing the allograft of the accumulating cellular infiltrates and fluid during rejection.