ras oncogene mutations and microsatellite instability (MIN) have been described in pancreatic cancer studies from paraffin blocks and fresh frozen tissue. We sought to determine whether they could be detected in endoscopic retrograde cholangiopancreatography-derived pancreatic juice. ras mutations were detected in the pancreatic juice of 40% (2 of 5) of patients with pancreatic cancer and 2 of 5 patients with pancreatitis. MIN was detected at a single locus in the pancreatic juice of 40% of pancreatic cancer patients and at > or = 2 loci of 100% of pancreatitis patients. The finding of MIN in pancreatitis specimens was verified in studies performed on paraffin blocks. MIN was not detected in normal pancreas controls. All of the cancer patients who had ras mutations in their pancreatic juice also had evidence of MIN at one or more loci (P < or = 0.05), suggesting that MIN is associated with the development of a ras mutation. More importantly, the finding of MIN in pancreatitis specimens suggests that MIN can occur in nonneoplastic conditions of the pancreas and may represent the saturation of an intact mismatch repair system.