The urine of 17 renal allograft recipients was examined for the presence of pyroninepositive lymphocytes for a period of nine months following operation. During the period of observation, 16 episodes of acute rejection occurred. In 14 of these episodes, pyronine-positive lymphocytes were observed in a spun sediment of the urine stained with methyl green pyronine. In 12 of the episodes of lymphocyturia, the cells appeared at least two days before the clinical diagnosis of acute rejection was made. Four of the rejection episodes occurred late after transplantation, that is, beyond four months, and in all of these episodes lymphocyturia was noted. It is concluded that routine examination of the urine for pyroninophilic lymphocytes is of value in the diagnosis of acute allograft rejection, and that serial examinations should be continued for at least nine months after operation.