Orthotopic transplantation of human tumors in nude mice reproduces the pattern of local growth and distal dissemination. The aim of our study was to determine the pattern of genetic alterations in human carcinomas of the exocrine pancreas orthotopically implanted and perpetuated in nude mice. Eight of the sixteen orthoimplanted human pancreatic carcinomas were perpetuated through several passages. Four perpetuated tumors followed distinct patterns of distal dissemination. Point mutations in the K-ras gene, genetic aberrations in the p53 and p16 genes, and allelic losses at retinoblastoma, adenomatous polyposis coli, and deleted in colorectal cancer loci were analyzed. Perpetuated tumors maintained the pattern of genetic alterations present in primary tumors. Five perpetuated tumors contained K-ras mutations, and all tumors contained p53 and/or p16 genetic aberrations. Allelic losses were present in four of the perpetuated tumors. Additional genetic alterations were detected in 6 of 35 metastases analyzed. Five of 9 peritoneal metastases or malignant ascitic cells acquired either K-ras or second p53 mutations. In contrast, only 1 of 25 liver metastases and none of the lymph node metastases acquired additional mutations. No additional p16 gene aberrations or other allelic losses were evidenced during tumor dissemination. We conclude that orthotopically implanted pancreatic carcinomas xenografted in nude mice show a high degree of genetic stability. Mutations in K-ras and p53 genes can occur in this model system in the more advanced stages of pancreatic tumor progression, mainly during peritoneal dissemination.