Thirty-one routinely processed radical prostatectomy specimens were examined for the presence of numerical chromosomal aberrations by in situ hybridization with centromeric nucleic acid probes specific for chromosomes 7, 10, 17, X, and Y. In eight of the prostatectomy specimens, chromosome numbers consistent with a normal male karyotype were found. Three cases, in addition to diploid chromosome numbers, showed a focal doubling of hybridization signals, consistent with tetraploidy. The other 20 cases displayed more or less marked numerical chromosomal aberrations. In this group, the appearance of numerical chromosomal aberrations often showed considerable local heterogeneity and was significantly correlated with tumor stage, Gleason grades, and tumor volume. We conclude that in prostatic cancer the presence of numerical chromosomal aberrations is associated with advanced disease. Especially in low differentiated tumors local heterogeneity in chromosome numbers can be very marked.