A 67-year-old patient presented with abdominal pain and distension 2 days after robotic radical prostatectomy for prostate carcinoma. He became anuric, and his serum creatinine level doubled, making IV contrast contraindicated. Abdominal CT without contrast demonstrated hypodense fluid in the peritoneum. Tc-MAG3 renogram detected extravasation of radiotracer from the bladder. Follow-up retrograde cystogram revealed a posterior anastomotic leak. The patient underwent uneventful surgical repair and made a full recovery. This case demonstrated that Tc-MAG3 can prove leak from the urinary tract, particularly helpful in the setting of poor renal function and contraindication to IV contrast.