Although detailed cytogenetic analysis has been carried out in many types of cancer, there is little information on the chromosomal makeup of prostatic cancer cells. Karyological analyses of cell lines derived from both metastatic and primary prostatic carcinoma have been carried out by Q-, C-, and sequential banding techniques. The metastatic line, PC-3, isolated from a bone marrow specimen, is an established epithelial line which is tumorigenic in nude, athymic mice and forms colonies in semisolid agar suspension. A subline, PC-3/M, was isolated from a PC-3-induced mouse tumor. Karyotypic analysis of PC-3 by Q- and C-banding showed the cells to be aneuploid at all culture passage levels. The modal chromosome number shifted from 62 to 55 between the 5th and 50th passages. PC-3 has a unique karyotype. Chromosomes 2, 3, 5, 15, and Y were always absent. At least 11 different marker chromosomes were observed. The subline, PC-3/M, had a similar karyotype and retained the parental PC-3 markers. PC-3/M had a more restricted chromosomal frequency distribution range. Nearly 73% of the PC-3/M cells examined had 60 or 61 chromosomes in contrast to the wide distribution seen in PC-3. Silver staining for nucleolus organizer regions indicated that the number of functional nucleolus organizer regions in PC-3 was proportional to the number of acrocentric chromosomes. Banding analysis of PC-5-PI isolated from primary prostatic adenocarcinoma indicated that this line also had a characteristic karyotype with 28% pseudodiploid and 72% pseudotetraploid components. All metaphases examined were partially trisomic in chromosome 9 and lacked a demonstrable Y chromosome.