Interphase cytogenetics by in situ hybridization (ISH) using a panel of centromere-associated DNA probes for chromosomes 1, 7, 9, 10, 11, 16, 17, and 18 was performed on 5 microns thick frozen tissue sections of transitional cell carcinomas (TCCs) of the urinary bladder. By this approach, chromosome ploidy, numerical chromosome aberrations, imbalance between chromosomes, and heterogeneity of aberrations within individual tumours were determined. In 15 of 24 TCCs, loss or underrepresentation of chromosome 9, compared with the ISH copy numbers of at least five other chromosomes, was demonstrated. Independently, RFLP analysis were performed on the same cases to detect loss of heterozygosity (LOH) of chromosome loci 9q34, 11p15, 16q22-24, 17p13, and 18q21. LOH was found in 9 of 19 informative cases for chromosome locus 9q34. Comparison of the ISH and RFLP results showed no correlation between numerical aberration and LOH for the loci on chromosomes 11, 16, 17, and 18. However, numerical loss of chromosome 9 was found in 89 per cent (eight of nine cases) with LOH for 9q34. Conversely, LOH at 9q34 was observed in only 67 per cent (eight of 12 cases) with underrepresentation of chromosome 9. Moreover, in 60 per cent of the non-informative cases (three of five cases), underrepresentation for chromosome 9 was observed. These results indicate that the heterochromatin probe for chromosome 9 can be reliably used in TCC tissue sections for the detection of chromosomal loss. In aneuploid TCCs, this DNA probe can be used for the detection of chromosomal underrepresentation only in combination with other centromere-associated DNA probes.