PURPOSE: Kidney size (KS) is used to diagnose the patients with renal disease. When the length of a kidney is measured under 9 cm, it is considered to indicate an irreversible disease. Because glomerular filtration rate (GFR) decreases with age, the normal range of KS in the elderly is indefinite. Therefore, we measured KS in adults older than 80 years old and investigated correlated factors. METHODS: One hundreds six adults (51 men, 55 women: mean age 83+/-0.3) without renal disease were included. Their serum creatinine (Scr) levels did not exceed 1.3 mg/dL, and the calculated GFR were over 60 mL/min/1.73m2. Abdominal ultrasonography were performed to all of them and their body indexes (BI) were measured. RESULTS: 1) The mean length of kidney was 9.9+/-0.07 cm. 2) KS in the early eighties was larger than that of adults over ninety. 3) KS showed negative correlations with age and Scr, but a positive correlation with body surface area . 4) The calculated GFR showed correlations with the surface areas of both kidney (BK) by C-G equation and with the size of BK by MDRD equation. 4) The GFR calculated by MDRD and C-G equation presented inverse correlations with Scr, but only MDRD equation showed a statistic significance. CONCLUSION:In the elderly, KS may be smaller than that of younger adults. Other factors such as either surface area or volume of BK and BI should be considered to estimate the individual KS to decide whether the size is within normal range.