Chromosome 1 abnormalities with loss of 1p36 have been investigated in 95 breast-cancer samples by means of a dual-target fluorescence in-situ hybridization (FISH) technique using the pUC 1.77 and p1-79 probes, specific for the 1q12 and 1p36 regions, respectively. Abnormalities for one or both probes were detected in 83/95 samples. Relative 1p36 under-representation was found in 79/95. The clinical relevance of these alterations was studied by comparing the FISH results with several parameters currently used in breast-cancer pathology. Distinct patterns of chromosome 1 abnormalities were found among the histologic types of breast carcinoma. Lobular or mucinous samples showed few or no alterations, whereas most ductal samples had high chromosome 1 polysomy with under-representation of 1p36. In ductal carcinomas, chromosome 1 alterations increased with histologic grade, DNA aneuploidy, loss of bcl-2 and high c-erb B-2 expression. These associations were found to be statistically significant. No correlation between chromosome 1 alterations and nuclear grade, age, size, lymph-node involvement, hormonal receptor presence, proliferation activity or p53 protein expression was detected. These results indicate the utility of this FISH technique for a better definition of the biological characteristics of ductal carcinomas.