BACKGROUND It is difficult to diagnose intermittent hydronephrosis and to decide the indication of surgical intervention. We investigated 23 cases of intermittent hydronephrosis. METHODS From 1978 to 1995, a total of 23 patients were diagnosed as intermittent hydronephrosis in our institution. We inspected their clinical features and treatment which had been performed to them. RESULTS Our study comprised 21 boys and 2 girls, whose mean age was 6 year old. Their chief complaint was intermittent flank pain (left: 21, right: 1, bilateral: 1) accompanied with gross hematuria (30%) and vomiting (39%). When they were asymptomatic, an excretory urogram revealed only mild pelvic dilatation without calyceal distension and kinking of ureteropelvic junction. Split renal function study by RI showed no difference between the affected side and the normal side except one case. When pelvic or calyceal enlargement was confirmed on ultrasonography while they were symptomatic, surgery was indicated. Surgery was performed in 17 cases (74%) including dismembered pyeloplasty in 14 cases, resection of aberrant vessel in 1, relocation of lower pole renal vessel in 1 and nephrectomy in 1. In surgical and histological view points, intrinsic stenosis was seen in 10 cases, extrinsic obstruction caused by aberrant vessels was seen in 4 and ureteral polyp was seen in 3 (bilateral polyp in 1 case). CONCLUSION They had no more symptoms after operation. Of 23 among followed up cases without surgery, we experienced 2 cases unexpectedly advancing irreversible hydronephrotic change after the last attack, 1 case of gradually progressing hydronephrotic change and 1 case of severe renal dysfunction after many attacks. Therefore intermittent hydronephrosis should be followed up carefully.