Endocrine neoplasms of the pancreas differ from the more common adenocarcinomas of the pancreas not only in histologic appearance, but also in clinical presentation and biologic behavior. Chromosomes were analyzed from nine fresh pancreatic endocrine neoplasms. Clonal chromosomal abnormalities were found in five; all were malignant neoplasms. One showed only a loss of the Y chromosome and another had a small triploid population of cells in addition to a normal mainline, with a karyotype of 61-66,XX, -X, -1, -2, -3, -4, +5, -6, +7, -11, -14, +17, +18, +20, +mar1,x2, +mar2,inc. Three neoplasms had near-haploid clones. One neoplasm had a composite karyotype of 31-36<n>,X, +1, +3, +5, +7, +9, +10, +17, +18, +mar. Two were from the same patient, who had the autosomal dominant syndrome MEN-1. The same clone, described as 29<n>,X, +add(1)(p12), +5, +7, +8, +18, +19, was found in both the primary pancreatic neoplasm and in the metastatic tumor. To our knowledge, this is the first report of a haploid clone in both a primary and metastatic solid tumor, and suggests that the near-haploid state is at least compatible with metastasis. These data, combined with the limited reports of cytogenetic data from endocrine pancreatic neoplasms, suggest that at least half of such neoplasms will have an abnormal karyotype.