Purpose: As bilateral ureteropelvic junction obstruction(UPJO) is expected to follow a different clinical course than unilateral UPJO, we compared the results of different treatment approaches to bilateral UPJO in an attempt to understand the natural course and the prognosis of the disease. Materials and Methods: Twenty six patients were selected among patients diagnosed with bilateral hydronephrosis during the last 7years, excluding those with grade II(GII) or less hydronephrosis in both kidney, those accompanied with VUR or other urinary tract anomalies and those who had received bilateral pyeloplasty. Seven patients underwent unilateral pyeloplasty and remaining 19 patients received conservative treatment with prophylactic antibiotics. Of 19 patients on conservative treatment, 2 patients received unilateral pyeloplasty due to no improvement or aggravation of hydronephrosis after more than 6months follow-up. Results: All patients were male and mean follow-up period was 13months(3-64months). On separate comparison of more severe and milder sides in both groups, surgical intervention group showed faster improvement than conservative group in both sides. Mean time to improvement to GII or less hydronephrosis was 5.7months and 5.0months for surgical intervention group and 16.1months and 13.1months for conservative group respectively for more severe sides and milder sides. At 6months follow-up, the ratio of improvement to GII or less was higher in surgical intervention group for both sides. Conclusions: Surgery and conservative treatment yielded comparable improvement, but in aspects of rate and ratio of improvement, surgery group can be expected to show faster and more reliable improvement. Early surgical correction should be considered in patients with bilateral UPJO in order to provide early protection to the contralateral and ipsilateral kidney, especially in age range with rapid maturation of renal function.