A 69-year-old female was admitted to our hospital in Oct 9, 2019 because of dry cough, dyspnea on exertion and fatigue for a month. She had no night sweats or weight loss. The patient was a nurse and she had neither previous disease nor history of smoking. She also had no history of exposure to foreign antigens inorganic particulates. In addition, she had no family history of rheumatologic or autoimmune diseases. On admission, temperature, pulse, respiratory rate, and blood pressure were 36.6 C, 79 beats/min, 21 breaths/min, and 126/88 mm Hg, respectively. Serum levels of electrolytes, brain natriuretic peptide, and liver & kidney function were normal. The levels of serum carcino-embryonic antigen (CEA), neuro-specific enolase (NSE), CYFRA 21-1, squamous cell carcinoma antigen (SCCA), and pro-gastrin-releasing peptide (Pro-GRP) were 1.24 ng/ml (0-5.0 ng/ml), 17.78 ng/ml (0-16.3 ng/ml), 1.54 ng/ml (0.1-3.3 ng/ml), 0.9 ng/ml (0-1.5 ng/ml), and 34.19 pg/ml (0-63 pg/ml), respectively. A computed tomographic (CT) scan of the thorax demonstrated left pleural effusions and linear opacity (Figure 1). The exudative, yellow fluid with the predominance of lymphocytes (85%) was detected by thoracentesis. The concentrations of CEA, NSE, CYFRA 21-1, SCCA, and Pro-GRP in pleural effusion were 0.62 ng/ml (0-5.0 ng/ml), 0.86 ng/ml (0-16.3 ng/ml), 11.95 ng/ml (0.1-3.3 ng/ml), 2.5 ng/ml (0-1.5 ng/ml), and 34.72 pg/ml (0-63 pg/ml), respectively. The levels of total protein, lactic dehydrogenase (LDH), glucose, and adenosine deaminase (ADA) in pleural effusion were 54.1 g/L (serum level in the same day was 66.80 g/L), 208 units/L (serum level in the same day was 200.0 units/L), 6.96 mmol/L (3.6-6.0 mmol/L), and value was 47.5 IU/L (<45 IU/L). PR3-anti-neutrophil cytoplasmic antibodies (ANCA) and MPO-ANCA were both negative in the serum. Cultures of the pleural effusion were negative for acid fast bacilli (AFB), fungi, and other organisms. A purified protein derivative (PPD) skin test was also negative. The cytological examination of exfoliated cells in pleural fluid revealed no malignant cells. To clarify the cause of pleural effusion, video-assisted thoracoscopy was performed and revealed a myriad of white nodules on both the visceral and parietal pleura, as well as on the diaphragm (Figure 2). Histological examination of the pleural biopsy sample demonstrated non-caseating granulomas and real-time quantitative polymerase chain reaction (PCR) for mycobacterium tuberculosis was negative (Figure 3). Stains for acid fast bacilli and fungi remained negative. These results, together with no evidence of other granulomatous disorders, were believed to be consistent with pleural sarcoidosis and this patient was started on prednisone 30 mg daily. All complaints regressed 2 weeks later and a follow-up CT 4 weeks later exhibited the left pleural effusion almost disappeared (Figure 4).