Estimation of renal length was carried out during radionuclide function studies, in cases of pediatric kidneys transplanted into adolescents or adults. In all 24 cases of single kidney transplants, the kidneys eventually increased in size, but at varying rates. From 5 cases with repeat studies close in time, and shortly after transplantation, an apparent maximal growth rate was estimated. In 3 pairs of dual renal transplants, with both kidneys surviving, marked differences in growth of each member of the pair was noted. The present initial description, of rates of change in size of the transplanted pediatric kidneys, may serve as a baseline from which to sort out the contributions of pre- and post-transplant factors.