There have been only a few reports of autoimmune pancreatitis complicated with pancreatic cyst and such cysts are rare, probably due to the absence of severe tissue necrosis and/or lack of stasis of the pancreatic juice in this condition. However, during a follow- up of 48 patients with this disease, we found 3 patients with pancreatic cysts, and this enabled us to evaluate their clinicopathological findings. Between September 1994 and July 2003, we treated and followed 48 patients with autoimmune pancreatitis, and found 3 patients with pancreatic cyst formation that was responsive to corticosteroid therapy. All of the patients with cysts had high serum IgG4 concentrations. After corticosteroid therapy, rapid resolution of the pancreatic cysts was observed. Immunostaining with goat polyclonal antibody for each IgG subclass showed severe infiltration of IgG4- posi- tive plasma cells in the cyst wall in one patient. The high serum IgG4 concentration and favorable response to corticosteroid therapy suggests that a highly active state of the inflammatory process is closely associated with cyst formation, and that a corticosteroid- responsive pancreatic cyst is a characteristic feature of autoimmune pancreatitis. There have been only a few reports of autoimmune pancreatitis complicated with pancreatic cyst and such cysts are rare, probably due to the absence of severe tissue necrosis and / or lack of stasis of the pancreatic juice in this condition. However, during a follow- up of 48 patients with this disease, we found 3 patients with pancreatic cysts, and this enabled us to evaluate their clinicopathological findings. Between September 1994 and July 2003, we treated and followed 48 patients with autoimmune pancreatitis, and found 3 patients with pancreatic cyst formation that was responsive to corticosteroid therapy. All of the patients with cysts had high serum IgG4 concentrations. After corticosteroid therapy, rapid resolution of the pancreatic cysts was observed. Immunostaining with goat polyclonal antibody for each IgG subclass showed severe infiltration of IgG4-posi- tive plasma cells in the cyst wall in one patient. The high serum IgG4 concentration and favorable response to corticostero id therapy suggests that a highly active state of the inflammatory process is closely associated with cyst formation, and that a corticosteroid-responsive pancreatic cyst is a characteristic feature of autoimmune pancreatitis.