Objective Hypertension and diurnal blood pressure (BP) variation are widely accepted as risk factors for renal damage. However, the effects of unilateral nephrectomy on BP and its circadian rhythm have not yet been clarified in patients with a compromised renal function, including dialysis patients. Methods We investigated 22 unilateral nephrectomized patients (16 men and 6 women, age: 64.5±14.3 years). The function of the circulating renin-angiotensin system (RAS) (plasma renin activity and plasma angiotensin II) and 24-h ambulatory BP monitoring (ABPM) were evaluated before and after nephrectomy. Daytime and nighttime 24-h ABPM values were determined based on sleep and waking times. Results In non-dialysis patients, the estimated glomerular filtration rate after nephrectomy was significantly lower than that before (before, 62.4±23.2 mL/min/1.73 m2 vs. after, 43.7±16.8 mL/min/1.73 m2; p<0.01). No significant differences were noted in the levels of BPs and circulating RAS before and after nephrectomy. However, the night-to-day (N/D) ratio of systolic BP (SBP) was significantly higher after nephrectomy than before (before, 93.3±6.5% vs. after, 98.4±6.9%; p<0.01), and the patterns of circadian BP rhythm also significantly differed before and after nephrectomy (p=0.022). Namely, the rates of dipper patterns decreased and nondipper and riser patterns increased after nephrectomy. In contrast, in dialysis patients, no significant differences were observed in the N/D ratio of SBP or the patterns of circadian BP rhythm before and after nephrectomy. Conclusion Unilateral nephrectomy affects the circadian rhythm of BP but not absolute values of BP.