We examined the magnitude of adaptive hyperfiltration in the remaining kidney of 16 aging (> 57 yr) and 16 youthful (< 55 yr) individuals who had undergone contralateral nephrectomy. Healthy volunteers who were youthful (N=143) or aging (N=37) provided control values for the binephric condition. One kidney GFR (+42%), renal plasma flow (+38%), plasma oncotic pressure (+2.8 mmHg), and mean arterial pressure (+7.0 mmHg) were all higher in youthful uninephrics versus binephrics. Corresponding excesses in aging uninephrics versus binephrics were by 38%, 36%, +1.4 and +14.0 mmHg respectively. Modeling of these data revealed that an isolated increase in either glomerular ultrafiltration coefficient (Kf) by 110% or in the transcapillary hydraulic pressure gradient (P) by 7 mmHg, could account for the observed level of hyperfiltration in youthful uninephrics. Corresponding increases for aging uninephrics were by 61% for Kf and 5 mmHg for P. We conclude that the magnitude of adaptive hyperfiltration is similar in aging to that in youthful uninephrics, albeit at a lower absolute GFR level. Isolated increases in either Kf or P or a combination of smaller increases in both can account for the hyperfiltration. Greater adaptive arterial hypertension in aging than youthful uninephrics raises the possibility of a disproportionate role for glomerular hypertension and P elevation in aging compared to youthful uninephrics. Glomerular hypertension could exacerbate the sclerosing glomerulopathy of senescence and lead to renal insufficiency. We recommend that living donors of a kidney transplantation in or beyond the seventh decade be used with caution.