No numerical aberration of chromosomes that might be specific for prostate cancer has so far been established. We used fluorescence in situ hybridisation (FISH) with centromere-specific probes for chromosomes 7, 8, 17, X and Y to establish the distribution of centromere copy numbers in frozen-stored or freshly prepared samples of benign prostate hypertrophy (BPH) and to detect numerical aberrations of these chromosomes in 28 prostate cancers from Japanese men. There was no significant difference in the data of centromere copy numbers between fresh and frozen-stored tissue. The most common aberration in prostate cancers was a gain of chromosome 8 (57%), with numerical aberration of chromosome 7 being the second most frequent anomaly (50%). Numerical aberration of chromosome 7 is most significantly associated with a higher Gleason score (GS) (P < 0.005) or with lymph node metastasis (P < 0.001). Numerical aberration of several chromosomes, including chromosomes 7 and/or 8, was common in aggressive prostate cancers. Loss of chromosome Y was detected in only 4% of cases. FISH analysis thus proved to be a useful method for detecting numerical aberrations of individual chromosomes, with application to touch preparations of frozen-stored tissue having the advantage of exact sampling of cancer foci. The results suggest that numerical aberration of chromosome 7 is associated with aggressive tumour behaviour and poor prognosis of patients with prostate cancer. The association between genetic change and chromosomal abnormality should be studied in detail.