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Serum immunoglobulin G (IgG) and/or IgG4 elevation is one of the notable characteristics of autoimmune chronic pancreatitis (AIP). The purpose of this study was to compare the sensitivity and specificity of IgG with those of IgG4 in the diagnosis of AIP.From December 2005 to March 2006, patients who were diagnosed as having ordinary chronic pancreatitis of a certain cause (n = 67) and pancreatic cancer (n = 76) in Asan Medical Center were enrolled. The IgG and IgG4 levels of these patients were compared with those of 35 AIP patients diagnosed in Asan Medical Center.The percentage of patients with serum IgG level more than 1800 mg/dL was 10.4% (7/67), 2.6% (2/76), and 54.3% (19/35) in patients with ordinary chronic pancreatitis, pancreatic cancer, and AIP, respectively. As for serum IgG4 levels more than 135 mg/dL, it was 11.9% (8/67), 1.3% (1/76), and 73.3% (22/30), respectively. The specificity of IgG at 1800 mg/dL and IgG4 at 135 mg/dL was both 93.7%. The serum IgG4 showed high specificity (98.7%) in differentiating AIP from pancreatic cancer. The IgG4 level at 141 mg/dL was determined as the most optimal cutoff value with resulting sensitivity and specificity of 73.3% and 95.1%, respectively (area under the curve, 0.816), whereas for IgG, it was determined as 1770 mg/dL, with sensitivity and specificity of 57.1% and 93.7% (area under the curve, 0.788).The sensitivity of serum IgG4 tended to be higher than that of IgG in the diagnosis of AIP. The IgG4 showed high specificity in the differential diagnosis of AIP from pancreatic cancer. Serum IgG4 should be included in the diagnostic workup for AIP.