A positive family history of prostate cancer is a risk factor for this disease, suggesting that alterations of certain genes may play an important role in the development and progression of prostate cancer. However, genetic alterations responsible for initiation and acquisition of metastatic phenotypes by prostate cancer are not well defined. We have observed a consistent change in chromosome 5 in an in vitro cell model of human prostate carcinogenesis in which the near-diploid cells from the surrounding tissue of an adenocarcinoma of the prostate obtained from a 42-year-old patient were subjected to in vitro cell culture and passages. We have examined three different passages of this cell strain by conventional and molecular cytogenetic methods and have seen an increased number of alterations in chromosome 5 in higher passage cells, with accompanying changes in cell morphology. In late passages of this cell line, no cell showed two normal copies of chromosome 5 as analyzed by G-banding and fluorecent in situ hybridization (FISH). The long arm (q) of chromosome 5 was either missing or involved in structural rearrangements. This observation suggests that the q arm of chromosome 5 may carry a tumor suppressor gene(s) that is well-expressed in normal prostate tissue, but when one of these tumor suppressor gene(s) is mutated or deleted and its encoded mRNA and protein are differentially expressed or not expressed at all in the prostate cells, then it may lead to initiation of tumor growth and development. Cytogenetic analyses of early passage cells in this cell strain revealed that approximately 78.8% of metaphases were normal, with a 46,XY chromosome constitution, and 21.2% of cells had clonal alterations mostly of chromosomes 5, 7, 8, 15, 16 and Y. In the middle passages, abnormal cells increased in number (78.26%) and also showed a large number of chromosomal changes. In the late passages, all cells showed structural and numerical abnormalities of the same chromosomes, in addition to some new markers; no cells were found to have a normal karyotype. These chromosomal aberrations could be considered early markers of prostate carcinogenesis. Some of the markers present in late passage cells were similar to those reported in a well-characterized prostate cancer cell line, LNCaP.