We investigated the relationship between changes in renal parenchymal volume (RPV) and renal function after a unilateral nephrectomy. In all 46 consecutive patients undergoing unilateral nephrectomy were included. Three-dimensional ultrasonography was performed before surgery (n = 46) and at 4.3 ± 1.9 h (n = 46), 2 days (n = 39), and 7 days (n = 43) after surgery to determine RPV. The mean RPV of the remaining kidney was 104.7 mL before surgery, 116.1 mL (+13.2%) at 4.3 h, 122.7 mL (+18.2%) at 2 days, and 117.6 mL (+13.3%) at 7 days after surgery. Sequential volume measurements during surgery (n = 5) revealed that the transection of the renal artery was followed by an increase in RPV on the contralateral side at 60−90 min. Multivariate regression analysis revealed that RPV was positively associated with the single-kidney glomerular filtration rate (GFR) and body surface area in both the preoperative and postoperative states. An increase in the single-kidney GFR was not seen at 2 years in patients with an RPV increase of <5% at 1 week (102.9 ± 8.4%, P = 0.322), but was still significant in the other patients (115.5 ± 21.0%, P < 0.001). These findings suggest that kidney volume may increase due to enlargement of the vascular bed caused by increased renal blood flow during the acute phase. Thereafter, histological hypertrophic responses may replace the hemodynamic changes in the chronic phase.