Urinothorax (pl. urinothoraces) means urine in the fluid-filled cavity that surrounds the lungs. It is a rare case of pleural effusion secondary to obstructive uropathy whereby urine forms a collection in the pleural cavity. The urine arrives in the pleural space either retroperitoneally under the posterior diaphragm, or via the retroperitoneal lymphatics. It remains a rare, possibly under-diagnosed, differential in the case of transudative pleural effusion.