BACKGROUND Autoimmune pancreatitis (AIP) is a unique form of chronic pancreatitis that is characterized by swelling of the pancreas, narrowing of the main pancreatic duct (MPD), elevation of serum gamma globulin, or immunoglobulin G or presence of autoantibody, lymphoplasmacytic infiltration and dense fibrosis on histopathology. It is responsive to steroid therapy. The incidence of AIP can reach 5-6% of chronic pancreatitis. It can present as obstructive jaundice, body weight loss, and pancreas head mass mimicking pancreatic cancer. The recognition of AIP can avoid major surgery such as pancreatic resection. METHODS From May 2003 to July 2007, a total of 5 cases of AIP were reviewed retrospectively. The diagnosis was made on imaging study, serology, steroid response and/or histology if surgery was carried out. RESULTS There were 2 male and 3 female patients, with a mean age of 61 (39-75) years. Atypical AIP was found in the first case and typical AIP in the remaining 4. The presenting clinical pictures were mild epigastric pain, obstructive jaundice, and loss of body weight in 4 cases, with associated autoimmune disease in 1. Diffuse or long segmental enlargement of the pancreas without peripancreatic fat infiltration was found in all patients except 1 who only had focal pancreatic head enlargement. Distal common bile duct (CBD) stricture was seen in 4 cases and the median CBD stricture length was 1.2 (0.5-2.5) cm. Multiple narrowing of the whole MPD was seen in 2 cases, focal narrowing of the MPD in 2 and long segmental narrowing of the MPD in 1. Serum immunoglobulin G tests were done in 4 cases and were elevated in all. Antinuclear antibody was positive in 3. The first case was operated on after a preoperative diagnosis of suspicious pancreatic head tumor. The subsequent 3 cases were diagnosed correctly as AIP. The last case presented with distal CBD stricture and hypoechoic lesion in the pancreas head on endoscopic ultrasound, with only borderline pancreatic enlargement on computed tomography, and he was operated on. Retrospective endoscopic retrograde pancreatogram review revealed MPD narrowing in the pancreatic body. Endoscopic retrograde brush cytology was performed and was negative for malignancy in 3 cases. Steroid therapy was given in 3 and was responsive, but there were 2 recurrences. CONCLUSIONS AIP should be a differential diagnosis in distal CBD stricture and pancreatic head mass when the patient has: (1) diffuse or long segmental enlargement of the pancreas without peripancreatic fat infiltration, with multiple narrowing of the MPD without much upstream dilatation, or narrowing of the MPD not corresponding to the region of CBD stricture; and (2) abnormal immunoserologic tests.