An 83-year-old man, who was a former smoker, with anti-ribonucleoprotein (RNP) antibody-positive combined pulmonary fibrosis and emphysema presented with a cough and dyspnea. A chest radiograph showed bilateral pleural effusions. His laboratory data showed proteinuria and elevated levels of anti-nuclear antibodies, anti-double strand DNA antibodies, and CA125, with decreased serum complement levels. Thoracentesis showed an exudative pleural effusion with an increased lymphocyte count and elevated CA125 levels. A thoracoscopic biopsy specimen showed proliferation of CA125-positive mesothelial cells. Systemic lupus erythematosus was diagnosed. His symptoms and pleural effusion resolved after the initiation of systemic corticosteroid therapy. The detection of anti-RNP antibody and CA125 levels are helpful in the diagnosis of lupus pleuritis.