To gain a better understanding of chromosomal aberrations in direct correlation with histology, we studied tumor material from 35 patients (36 regions) with primary prostate carcinoma by nonisotopic in situ hybridization. Nine biotinylated DNA probes were used on serial paraffin sections (centromer-specific probes for X, Y, 1, 7, 8, 10, 17, and 18, and a telomer-specific probe for 1p; ONCOR). Of the 324 hybridized sections, 94% were suitable for evaluation. In 34 of the 35 cases (35 of 36 regions) 1-8 chromosomal aberrations were detected. Chromosome X showed supernumerary centromer copies in 44% of cases. The probes for chromosomes 1, 1p, 10, and 18 demonstrated deletions in 25, 23, 40 and 58% of cases, respectively. Gains as well as deletions were present for Y, 7, 8, and 17 in 31, 25, 36, and 58% of cases, respectively. In 27% of cases discordant copy numbers of the centromer- and the telomer-specific probes for chromosome 1 were observed. No aberration which might be specific for prostate cancer could be established. The rate of aneusomy increased significantly with histological grade. Intratumoral heterogeneity of chromosomal aberrations was revealed in one case. Due to the higher sensitivity of nonisotopic in situ hybridization, aneusomic cases outnumbered cases with cytometrically determined DNA aneuploidy. In view of published results of metaphase preparations, the high frequency of aneusomy and some of the chromosomal aberrations detected by nonisotopic in situ hybridization were unexpected.