Cytogenetic analysis after short-term culture in vitro of primary tumor samples was attempted in 82 patients with prostatic cancer. Tumor material was obtained by radical prostatectomy o r transurethral resection. Successful cytogenetic studies were performed on 57 tumors of which five were well, 30 moderately, and 22 poorly differentiated adenocarcinomas. Only normal karyotypes were found in 24 tumors. Structural nonclonal aberrations were detected in I 8 and clonal karyotypic abnormalities in I 5 tumors. The most common clonal numerical aberration was loss of the Y chromosome; a missing Y was found in six tumors, in three of these as the sole anomaly. Clonal structural chromosomal rearrangements, usually accompanied by numerical changes, were detected in I 2 tumors. The rearrangements involved I 8 of the 22 autosomes and the X chromosome. Chromosomes I, 7, and I0 were most frequently affected. Deletions, duplications, inversions, insertions, and balanced as well as unbalanced translocations were represented. The breakpoints in chromosome I were scattered along both the short and long arms with no obvious clustering, whereas those in chromosomes 7 and I0 were clustered a t bands 7q22 (two deletions and two duplications in four different tumors) and I Oq24 (two translocations, one deletion, and one inversion in four tumors). One additional tumor displayed a derivative chromosome 10 with a breakpoint in IOq23, and one had monosomy 10. Altogether, these abnormalities resulted in loss of IOq24+qter in five tumors. Monosomy 8 and rearrangements of the short arm of chromosome 8 leading to loss of 8p2I-pter were seen in four tumors. Double minute chromosomes were found in two tumors. Genes Chrom Cancer 4:16-24 (1992).