The goals of this study were to detect the numerical alterations of chromosomes 1 and 7 in breast cancer and to correlate the findings with DNA ploidy status as well as with parameters of prognostic significance. Fluorescence in situ hybridization (FISH) with centromeric probes for chromosomes 1 and 7 and cellular DNA content measurement by image analysis-based cytophotometry were applied on interface nuclei from fresh tissue imprints of 59 breast ductal carcinomas. Immunohistochemical stainings for estrogen receptor (ER), progesterone receptor (PR), HER-2, p53, and Ki67 were performed on paraffin tumor sections. The correlation between DNA ploidy and chromosomal aberrations revealed a significant association between aneuploidy and aneusomy for both chromosomes 1 (p=0.002) and 7 (p=0.00001), however, a number of diploid tumors were found to be aneusomic, especially for chromosome 1. Chromosome 7 polysomy was significantly associated with a higher incidence of axillary lymph node metastasis (p=0.05), poorly differentiated (grade III) tumors (p=0.03), negative ER and PR status (p=0.02 and 0.001, respectively), as well as p53 protein expression (p=0.05) and a higher Ki67 labeling index (p=0.004). Chromosome 1 aneusomy was only related with HER-2 protein overexpression (p=0.05). No association between chromosome alterations and tumor size was detected. In conclusion, the results of our study indicate that the detection of numerical aberrations of chromosomes 1 and 7 by FISH seems to be more sensitive than DNA ploidy status for the evaluation of abnormal cellular DNA and chromosome 7 aneusomy characterizes tumors with aggressive features and therefore might be a useful predictor of unfavorable biological behavior in breast cancer.