Two cases of neonatal urinary ascites are reported, one (case 1) caused by posterior urethral valves associated with right vesicoureteral reflux, and the other (case 2) secondary to bilateral obstructed megaureter--a very rare cause. Abdominal distension, electrolyte imbalance, and an elevated BUN/serum creatinine ratio were noted at the time of hospitalization in both cases. These laboratory values promptly returned to normal after the establishment of appropriate urinary drainage, accomplished through bladder catheterization in case 1 and bilateral percutaneous nephrostomy in case 2. In case 1, transurethral resection of the posterior urethral valves was followed by resumption of normal urination. In case 2, bilateral ureteroneocystostomy with ureteral tapering was performed successfully. These are only the fifth and sixth cases of neonatal urinary ascites reported in the Japanese literature.