Urinothorax, the presence of urine in the pleural space, is a rare cause of pleural effusion, usually associated with obstructive uropathy, or urinary trauma. We present the case of a 3 year-old boy and a systematic review of the literature of the 44 cases encountered. After resection of a Wilm's tumour in the right kidney our patient presented acute respiratory distress associated with radiographically confirmed pleural effusion. With the initial diagnosis of pneumonia or malignant pleural effusion, a closed thoracotomy was performed. The liquid obtained suggested urine, which was confirmed by the laboratory. Cystoscopy with retrograde pyelography detected a fistula on the posterior wall of the right kidney. The report of cases worldwide is low, probably due to its low incidence but also to underdiagnosis. Respiratory symptoms are not always present and urological symptoms usually predominate. Diagnosis requires a high degree of clinical suspicion and is confirmed by the main biochemical marker: The ratio >1.0 pleural fluid creatinine and creatinine serum.