Urinothorax ("pl". urinothoraces) means urine in the fluid-filled cavity that surrounds the lungs. It is a rare cause 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.

Handa et al., described 47 cases between 1967 and 2007, noting that it was more prevalent in males, generally ipsilateral to the obstruction, and in most cases relieved by clearance of the obstruction.

A pleural effusion is excess fluid that accumulates in the pleural cavity, the fluid-filled space that surrounds the lungs. This excess can impair breathing by limiting the expansion of the lungs. Various kinds of pleural effusion, depending on the nature of the fluid and what caused its entry into the pleural space, are hydrothorax (serous fluid), hemothorax (blood), urinothorax (urine), chylothorax (chyle), or pyothorax (pus). A pneumothorax is the accumulation of air in the pleural space, and is commonly called a "collapsed lung."

Various methods can be used to classify pleural fluid.

By the origin of the fluid:

- Serous fluid (hydrothorax)

- Blood (haemothorax)

- Chyle (chylothorax)

- Pus (pyothorax or empyema)

- Urine (urinothorax)

By pathophysiology:

- Transudative pleural effusion

- Exudative pleural effusion

By the underlying cause (see next section).