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Serum IgG4 level is a useful diagnostic marker of autoimmune pancreatitis (AIP); however, it is difficult to predict relapse. We investigated whether a change in serum IgG4 is predictive of AIP relapse during maintenance therapy.Seventy-three patients with AIP treated with steroids were divided into 3 groups according to their initial serum IgG4 levels: level 1 group (>2-fold upper limit), level 2 group (1- to 2-fold upper limit), and a normal group. The relapse rate and the prevalence of extrapancreatic lesions were compared between the 3 groups. The correlation between the relative rise of serum IgG4 levels and relapse during maintenance therapy was analyzed.There were no differences in serum IgG4 levels in the presence or absence of relapse. The average number of extrapancreatic lesions was 1.02, 0.47, and 0.27 in level 1, level 2, and the normal group, respectively. The relative rise of serum IgG4 levels after steroid therapy was significantly higher in relapse than in nonrelapse cases. The area under the receiver operating characteristic curve of the relative rise of IgG4 levels for discriminating between the relapse and the nonrelapse cases was 0.85.The relative rise of serum IgG4 levels after steroid therapy may provide an indication of relapse.