Introduction: Autoimmune pancreatitis (AIP) is characterized by diffuse pancreatic enlargement, main duct abnormalities, elevated serum gamma-globulins or IgG levels, autoantibodies, other autoimmune diseases (Sjo-gren's syndrome, primary sclerosing cholangitis), a periductal lymphoplas-macytic infiltrate, and responsiveness to corticosteroids. We describe a patient with AIP presenting with a large distal peripancreatic mass that relapsed after steroid withdrawal, developing IgG4 antibodies during relapse, though seronegative at presentation. Case: A 65-year-old Caucasian male presented with bloating and painless jaundice. CT and MRI revealed diffuse pancreatic enlargement with a distal mass. EUS-guided biopsies revealed chronically inflamed pancreatic parenchyma. Repeated ERCPs showed evanescent strictures in the distal common bile duct, hilum and pancreatic duct, with negative brushings. Tumor markers, gammaglobulins and autoimmune serologies were unremarkable. Prednisone was started for presumed autoimmune cholangiopancreati-tis, with marked clinical and radiographic improvement. Six months after steroid cessation, the patient had bloating and weight loss. Repeat imaging revealed a large hypodense peripancreatic soft tissue mass with vascular encasement, biopsies again revealing chronic inflammation and fibrosis, with new elevation of serum IgG4. He was restarted on prednisone, with dramatic mass reduction after 6 weeks, at which time azathioprine was initiated to allow a steroid taper. Discussion: The presentation of AIP with obstructive jaundice, weight loss, new-onset diabetes and pancreatic mass in the elderly male often mimics that of pancreatic adenocarcinoma. In reports from Japan, AIP is almost universally associated with autoantibodies (antinuclear antibody, rheumatoid factor, antilactoferrin antibody or anticarbonic anhydrase II antibody), with majority having hypergammaglobulinemia and isolated IgG4 elevation. In the West, seropositivity is less well described. Diabetes mellitus is present in 50%, which may improve after AIP treatment. CT or MRI shows a characteristic capsule-like rim around the inflammation, surrounding the pancreatic head in 80%, with calcifications or pseudocysts rarely seen. Patients typically have a prompt response to steroids, but treatment length, utility of other immunosuppressants and prognosis are not well known.