Fluorescence in situ hybridization (FISH) was used to study numerical and structural chromosome 1 aberrations in interphase nuclei of transitional cell carcinomas (TCCs) of the urinary bladder. One of the characteristic numerical aberrations, as detected previously in low-grade noninvasive TCCs, included trisomy for chromosome 1 (A. H. N. Hopman et al., Cancer Res., 51: 644-651, 1991). We examined in more detail 22 cases with a centromeric (1q12) and a telomeric associated (1p36) DNA probe and with a library DNA probe from sorted human chromosome 1 in single- and double-target FISH procedures. All flow cytometrically determined DNA diploid TCCs (13 cases), which showed three spots for 1q12 (6 cases), had two spots for 1p36. Since the library DNA probe showed three separate domains in the nuclei of these cases, the additional copy for 1q12 could be explained as an extra chromosome 1p-, containing the 1q12 target. In the flow cytometrically determined DNA tetraploid/aneuploid tumors, the results were more complex. In 6 of 9 cases, we observed an overrepresentation of 1q12 as compared to 1p36, also suggesting the presence of extra copies of 1p- chromosomes. The results of the present study demonstrate the utility of the FISH method to assess structural chromosome aberrations in interphase nuclei of solid tumors.