Urinothorax should be considered when pleural effusion occurs in patients with urinary tract obstruction accompanied by retroperitoneal urinoma. This recommendation is based on our experience with 4 cases and a review of the literature. In 2 patients urinary obstruction was owing to retroperitoneal fibrosis secondary to malignancy, including one who had a neuroectodermal undifferentiated small round cell tumor and the other who had a metastatic epithelial neoplasm. In the 2 other patients urinary obstruction resulted from a failed ureteroneocystotomy following renal allograft transplantation. All 4 patients had a urinoma, which may be a predisposing factor to urinothorax.