Cytogenetic and molecular studies demonstrated that pancreatic cancer frequently shows specific chromosomal abnormalities, such as losses of 9p, 17p, and 18q, and gains of 8q and 20q. We have analyzed alterations in the copy number of specific chromosomal regions in cells from the pancreatic juices of 32 patients with various pancreatic disorders by fluorescence in situ hybridization (FISH) technique to pursue the possible clinical use of early diagnosis of pancreatic cancer. None of the chromosomal abnormalities were found in 13 specimens from individuals who had no neoplastic lesions. On the other hand, 12 specimens (63%) derived from the remaining 19 patients who had neoplastic lesions showed at least one chromosomal abnormality. Ten of these specimens were from pancreatic cancer patients; 7 cases (70%) showed chromosomal abnormalities. All but one of the 12 tumors with chromosomal abnormalities had loss of 18q. Furthermore, we detected a tumor in one patient in whom the routine cytological method and endoscopic retrograde chorangiopancreatography found nothing. Based on the results by FISH, we performed endoscopic ultrasonography and found a small serous cystadenoma in this patient. These results indicate that: (a) FISH analysis of cells from pancreatic juices obtained during endoscopic retrograde chorangiopancreatography is quite useful for detecting pancreatic ductal tumors; and (b) loss of chromosome 18q is one of the early genetic changes that provide very useful information in diagnosing pancreatic neoplasias.