The urinothorax is an uncommon cause of pleural effusion; it involves the presence of urine in the pleural space, generally associated with bilateral obstructive uropathy or trauma of the urinary system. The biochemical characteristics of the effusion are related with to those of urine and they suffer a slight, but substantial modification as it passes into the pleural space where it mixes with the pleural fluid (PF). Characteristically, the PF is a transudate, with a pH < 7,3; however there have been publications where it has been classified as an exudate, associated with high LDH levels. In general, glucose and proteins levels are low (glucose levels < 60 mg/dl), while LDH levels are very high. The accepted biochemical parameter for the diagnosis of urinothorax is a ratio higher than 1 between pleural fluid creatinine and serum creatinine. To date, only few cases have been reported worldwide, probably due to misdiagnosis, to a low index of suspicion and the quick resolution of the pleural effusion with the removal of the urinary tract obstruction. In this study we review one case of urinothorax in a patient with bilateral obstruction of the urinary system by bladder cancer. She was admitted to the Internal Medicine Service of the Hospital Militar Central in Bogota. It is important that physicians familiarize themselves with this pathology to be able to establish an early diagnosis and provide adequate treatment.