BACKGROUND: Children with a solitary functioning kidney (SFK) are at risk for hypertension and kidney injury despite compensatory renal growth. However, the relationship between compensatory enlargement and blood pressure abnormalities remains controversial. This study is an exploratory observational study aimed at assessing ambulatory blood pressure monitoring (ABPM) parameters in children with an SFK and to evaluate the relationship between compensatory kidney enlargement and blood pressure indices. METHODS: Thirty-three children aged 6-18 years with an SFK (22 renal agenesis, 11 atrophic kidneys) were evaluated. Compensatory enlargement was defined as kidney length >97.5th percentile for height. All participants underwent anthropometric assessment, serum creatinine measurement, estimated glomerular filtration rate (GFR) calculation, and 24-hour ABPM. ABPM values were expressed as standard deviation scores (SDS) adjusted for age and sex. RESULTS: Compensatory enlargement was present in 17 (51.5%) patients. Those with compensatory enlargement showed significantly higher 24-hour systolic BP SDS (0.64±1.07 vs. -0.22±1.03, p=0.024), nighttime systolic BP SDS (1.36±1.04 vs. 0.38±0.93, p=0.008), nighttime diastolic BP SDS (1.42±1.17 vs. 0.48±0.70, p=0.009), and nighttime MAP SDS (1.43±0.99 vs. 0.48±0.84, p=0.006). Kidney length SDS correlated positively with 24-h systolic BP SDS (r=0.44, p=0.01) and nighttime MAP SDS (r=0.45, p=0.009). Logistic regression analysis revealed that compensatory enlargement independently predicted elevated blood pressure (OR 10.06, 95% CI 1.03-97.77, p=0.047) after adjustment for age, height SDS, and weight SDS. CONCLUSIONS: Compensatory hypertrophy in SFK may not be entirely benign and could reflect an adaptive process associated with altered blood pressure regulation. Higher nocturnal blood pressure in patients with compensatory enlargement may suggest subclinical hemodynamic stress. These findings suggest that children with compensatory kidney enlargement may exhibit subtle alterations in ambulatory blood pressure patterns, warranting further investigation in larger longitudinal studies.