Acute experimental hydronephrosis induces increased output of renin from ipsilateral kidney. In most clinical presentation of unilateral hydronephrosis the patients are however, normotensive with normal circulating plasma renin activity. In consideration of this argument we undertook this prospective study of ipsilateral renal vein renin activity in children with congenital Pelvic Ureteric Junction Obstruction (PUJO) and compared with other wise normal children. In this prospective cross-sectional study, started from July 99 to June 2001, twenty patients, age upto 12 years, with unilateral hydronephrosis and in control group ten patients without hydronephrosis were taken. 2ml blood was taken from the renal vein in study group and from infrarenal portion of inferior vena cava during exploratory laparotomy from the control group. All the patients had advanced grade of hydronephrosis and two patients (13%) had mild hypertension. In this study the mean plasma renin activity (PRA) was 45.58 ng/ml/hr (range: 11.69-67.56 ng/ml/hr) in study group. The mean PRA in control group was 5.9ng/ml/hr. The result of study group was significantly higher than normal (P value 0.0003). In Bangladesh we are doing more conservative kidney preserving surgery for PUJO, but need long term follow up of the patients undergoing surgery for PUJO in childhood for potential of developing renin-angiotensin induced hypertension in later life.