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Background: The most commonly isolated organisms in a parapneumonic effusion include S. pneumoniae, H. influenzae, and S. aureus. If unusual organisms are isolated from the pleural space, further investigation is warranted to locate the primary source. We present a patient with an infected chronic renal cyst found to have an empyema secondary to Proteus mirabilis to highlight the importance of further diagnostic workup when encountering unusual organisms in the pleural space., Case Presentation: A 40-year-old African-American female, with a past medical history of asthma and sickle cell trait, presented with 5 weeks of upper respiratory tract symptoms and chest pain. A computed tomography angiogram (CTA) of the chest was negative for a pulmonary embolism but revealed a loculated left sided pleural effusion with associated left-lower lobe consolidation. She was started on empiric antibiotics, and a chest tube was inserted with drainage of frank pus. Fluid gram stain was positive for gram negative rods. Intrapleural fibrinolytics were administered for 72 h given the presence of loculations. With no improvement following fibrinolytics, she was taken to the operating room for large bore chest tube placement and left visceral pleura decortication. Pleural fluid cultures speciated to Proteus mirabilis, so further cross-sectional imaging of her abdomen/pelvis was pursued to evaluate for a primary source. A complex cystic lesion in the upper pole of the left kidney that communicated with the ipsilateral diaphragm was identified. Subsequent drainage and culture of the renal cyst was positive for Proteus mirabilis. Given clinical improvement following these interventions she was discharged with an extended course of antibiotics with plans for repeat imaging following completion of treatment., Conclusions: While cases of Proteus mirabilis empyema have previously been reported as a consequence of conditions such as pyelonephritis, we present, to our knowledge, the first case of a Proteus mirabilis empyema as a consequence of an infected renal cyst communicating with the pleural space. This study highlights that further evaluation with cross-sectional imaging is warranted when unusual organisms are found in the pleural space. Anatomic abnormalities that become apparent on imaging may help elucidate the source of infection.