A 78-year-old man with cryptogenic chronic bilateral lymphoplasmacytic pleuritis, diagnosed based on left parietal pleural biopsy specimens obtained by pleuroscopy, developed acute left bacterial pleuritis. The left pleural effusion was neutrophil dominant, however, the right pleural effusion showed lymphoplasmacytic infiltration. Laboratory examinations revealed that his serum IgG4 concentration was increased, with a higher level of IgG4 in the right pleural effusion. Re-evaluation of the previous biopsy specimens using an immunostaining method revealed numerous IgG4-positive plasma cell infiltrations with IgG4-positive/IgG-positive plasma cells at 85.4%. Accordingly, the new diagnosis of this patient was considered to be chronic bilateral IgG4-related pleuritis.