A 76-year-old man presented with the chief complaints of generalized malaise and precordial discomfort. Clinical examination revealed evidence of pericardial and pleural effusions. Under microscopic examination, the pleural fluid was shown to contain abundant lymphocytes and histiocytes, including many clusters of differentiated CD4-positive T-lymphocytes. Cells with emperipolesis that appeared to be positive for S-100 protein expression were also detected. A pleural biopsy performed two months later revealed proliferating eosinophils. There was also marked lymphocyte and plasma cell infiltration and fibrosis, and based on elevation of the serum IgG4 levels, the diagnosis of IgG4-related pleuritis was made. This condition needed to be differentiated from Rosai-Dorfman disease (RDD) in the patient. Accordingly, this case study provides valuable insights into the pathology of IgG4-related diseases and Rosai-Dorfman disease.