Bilateral pleural effusion and pericardial effusion were seen at the second admission (B). lymph nodes, can be affected by IgG4-RD (3). While the lungs are involved in 9-18% of IgG4-RD patients (4-7), pleural involvement is observed in only 4% (4,5). Pleural effusion is uncommon, but previous reports have shown that it is usually exudative (8). Chylothorax, which is characterized by milky-appearing pleural fluid with elevated triglyceride levels or the presence of chylomicrons, is caused by the extravasation of chyle into the pleural space due to obstruction or damage of a thoracic duct or its tributaries or transdiaphragmatic flow from the peritoneal cavity (9). The etiologies of chylothorax include several causes, such as trauma (surgical or non-surgical), malignancy, lymphatic disorders, infection, chylous ascites, and other miscellaneous causes (10); however, chylothorax due to IgG4-RD has almost never been reported. We experienced a rare case of IgG4-RD with refractory chylothorax that was successfully treated with high-dose prednisolone (PSL) and rituximab (RTX). We report this case with a review of previous PI4KB case reports of IgG4-related pleuritis. Case Report A 66-year-old Japanese man with a history of pollen allergy and thyroidectomy for Graves-Basedow disease was admitted to another hospital with a 2-month history of leg edema, eyelid edema, and dyspnea on exertion. Computed tomography (CT) demonstrated pleural and pericardial effusions, and a pericardiocentesis revealed the fluid as a nonspecific inflammatory effusion with increased numbers of lymphocytes without any infection. Increasing the levothyroxine dose for latent hypothyroidism and initiation of furosemide therapy did not decrease the effusion. He was transferred to our department. At his first admission to our hospital, whole-body CT demonstrated pericardial effusion, bilateral pleural effusion, and testicular hydrocele. No swelling of the lacrimal or salivary glands nor pancreatic enlargement was observed. The right pleural effusion was exudative with a total cell count of 2,410/mm3 (lymphocytes, 75%) and neither malignant cells nor bacteria. Serum anti-SS-A/Ro antibody was slightly positive (15.4 U/mL; normal range, <10.0 U/mL) on an enzyme-linked immunosorbent assay but negative with the double immunodiffusion method. Other autoantibodies, including anti-SS-B/La, anti-CCP, anti-dsDNA, anti-RNP, anti-Scl70, and anti-neutrophil cytoplasmic antibodies, were all negative. Sialometry showed a rate of 1 1.008 mL/minute (within normal range), while salivary gland scintigraphy showed a slightly decreased uptake and secretory function. A lip biopsy demonstrated grade 2 lymphocytic infiltration according to Greenspan's classification (11), with only a few IgG4-positive plasma cells. The Schirmer test and rose bengal dye staining test were positive only in the left eye. Although he did not meet the ACR 2012 classification criteria (12), we suspected Sj?gren syndrome with serositis. PSL 40 mg/day (0.5 mg/kg) was initiated. Both the pleural and pericardial effusion amount decreased; however, tapering of the PSL led to exacerbation of the pleural effusion. At his second admission, he presented with swelling of the lacrimal glands. Laboratory data are shown in Table 1. The white blood cell count in the peripheral blood was Oleuropein 8,200/L (neutrophil 87.7%, lymphocyte 8.7%). Serum IgG and IgG4 Oleuropein levels were 1,500 mg/dL and 264 mg/dL, respectively. Autoantibodies were all negative. Serum levels of C-reactive protein (CRP), soluble interleukin-2 receptor (IL-2R), free T3, free T4, and thyroid-stimulating hormone (TSH) were also within the normal ranges (0.04 mg/dL, 394 U/mL, 2.9 pg/mL, 1.7 ng/dL and 1.67 IU/mL, Oleuropein respectively). Chest CT showed marked bilateral pleural effusion with passive atelectasis and slight pericardial effusion (Fig. 1B). Thoracentesis for the right pleural effusion (Table 1) revealed turbid yellow fluid (Fig. 1A) with a total cell count of 810/mm3 (lymphocytes 84%), total protein 5.7 g/dL, adenosine deaminase (ADA) 25.1 U/L, total cholesterol 84 mg/dL, triglyceride 300 mg/dL, and the presence of chylomicrons, Oleuropein compatible with chylothorax. A cytologic examination was negative for malignancy. General bacterial and mycobacterial cultures of the pleural fluid were negative. He had no history of trauma or thoracic surgery. Lymphangiography did not show any leakage or obstruction of the thoracic duct (Fig. 2). While thoracoscopy did not reveal any tumor, amyloid deposits, or leakage from the thoracic duct, a thoracoscopic surgical pleural biopsy demonstrated infiltration by lymphocytes and plasma cells with ectopic germinal centers under the pleural mesothelium. Approximately 50% of IgG-positive plasma cells were IgG4-positive (Fig. 3). Storiform fibrosis and obstructive phlebitis were not found in this small specimen..