This prospective study aims to evaluate the T1 relaxation time of autoimmune pancreatitis (AIP) in the native state and after corticosteroid treatment (CST). Thirty-four patients with AIP and twenty control subjects received pancreatic MR including T1 mapping. All AIP patients had T1 mapping data before and after CST. It turned out that the inflamed pancreatic parenchyma had significantly elongated T1 relaxation time, and after 4-12 weeks of CST, the T1 relaxation time shortened significantly towards normalization, in keeping with the serum biomarkers of disease activity. Therefore MR T1 mapping is a noninvasive, quantitative method to monitor AIP treatment effect.