Type 1 autoimmune pancreatitis (AIP) is a pancreatic manifestation of IgG4-related disease (IgG4-RD) and is a unique chronic inflammatory disease characterized by fibrosis. IgG4-RD is caused by an autoimmune mechanism that mimics malignant tumors and inflammatory disorders. Apoptosis inhibitor of macrophages (AIM) can function as a biomarker of autoimmune or inflammatory diseases with tissue fibrosis, including in inflammatory bowel disease and chronic liver disease. Therefore, the aim of the present study was to clarify the role of serum AIM levels and the clinical characteristics of patients with IgG4-RD and AIP. For this purpose, serum AIM concentrations were assessed using ELISA and the association between AIM and the laboratory and clinical data from patients with IgG4-RD/AIP, patients with pancreatic cancer (PC) and healthy controls (HCs), was determined. The results demonstrated that the serum AIM concentrations were not associated with the laboratory data. However, the serum AIM levels were significantly elevated in patients with AIP compared with the HCs and patients with PC. Furthermore, the serum AIM levels significantly decreased following steroid therapy in patients with AIP who were in remission. Overall, the present study demonstrates that serum AIM levels may be a potentially useful biomarker for the differential diagnosis of AIP and for evaluating the therapeutic reactivity of affected patients.