The relatively common occurrence of ureteral obstruction in patients with known malignancy elsewhere presents an interesting clinical problem. In those who have undergone abdominal surgery or radiation therapy for malignancy, it is difficult to determine whether ureteral obstruction is the complication of therapy or of recurrent metastatic carcinoma. Transperitoneal thin-needle aspiration biopsy provides an acceptable alternative in determining the true cause of the obstruction. This procedure is competitive with explorative surgery and laparoscopy and is much more specific than CT, excretory urography, and other imaging modalities. In our series of 32 patients, biopsy revealed metastatic tumor in 18 patients, thus altering their clinical management.