To clarify the frequency and cause of acute pancreatitis following hematopoietic stem cell transplantation (SCT), we examined retrospectively 57 patients who underwent hematopoietic SCT in our institute from 1984 to 2000. Twelve (21%) of the patients showed an elevated level of serum pancreatic amylase following SCT. However, only 3 patients were clinically diagnosed as having acute pancreatitis. Among these 12 patients, 11 had undergone allogeneic transplantation. Furthermore, patients who had undergone unrelated transplantation (7/16; 44%) tended to show a higher incidence of increased amylase than those who had undergone related transplantation (4/24; 17%). Six patients were at an advanced stage of acute GVHD (grade III or IV) and all showed an elevated level of serum amylase, whereas only four patients showed an elevated serum amylase level among 34 with mild acute GVHD (grade I or II) or without GVHD. Furthermore, five out of 12 patients who showed an increased amylase level were concurrently diagnosed as having viral infection such as cytomegalovirus, adenovirus, or varicella zoster virus. We conclude that pancreatitis following SCT occurs more often than realized, and is mostly subclinical. This is closely associated with severe acute GVHD, and possibly viral infection.