CASE PRESENTATION: 83 year old female presents with recurrent right sided pleural effusion. In the past two years the patient had undergone three thoracentesis which had revealed clear transudative effusions. Her history was notable for a pacemaker placement complicated with hemothorax ten years earlier and multiple uncomplicated cystoscopic surgeries and urinary tract instrumentation for urinary incontinence. After chest radiograph revealed moderate right size pleural effusion thoracentesis was performed revealing 1220 mL yellow clear fluid with pH of 7.47, pleural to serum protein ratio of 0.32, pleural to serum LDH of 0.28 consistent with transudate. Pressure volume curve during the thoracentesis revealed biphasic curve (Img 2), suspicious for dual process of trapped lung and additional cause of transudative effusion. Pleural fluid creatinine was 1.2 and the pleural to serum Cr ratio was 1.1, raising suspicion of urinothorax. This suspicion was further investigated with a 99Tm radionuclide renal scintigraphy scan which suggested communication between the right kidney and right pleural space.(Img 1) A CT was performed, showing visceral pleural lung thickening, consistent with trapped lung.