The TOP2A gene is located on chromosome 17 close to the HER-2 gene. It encodes an enzyme involved in the regulation of cell proliferation. Using fluorescence in situ hybridization (FISH), we have examined fine needle aspiration smears from 42 cases of breast carcinoma with probes for TOP2A, HER-2 and chromosome 17. We found that amplification of TOP2A is a frequent finding in breast cancer and is often but not exclusively accompanied by HER-2 gene amplification. It is associated with high histological grade and oestrogen receptor (ER) negativity. TOP2A deletions may also be associated with high histological grade and loss of ER. TOP2A amplification in the absence of HER-2 amplification may be associated with lower histological grade and ER positivity. Testing for TOP2A aberrations may be useful in the search for individually tailored treatment regimes for breast cancer.