Breast cancer is a genetically complex disease. Fluorescence in situ hybridisation can be used to analyse the genetics of breast-cancer progression in interphase cytogenetics. We have analysed the histological distribution of erbB2 and topoll alpha co-amplification in paraffin sections of invasive breast cancer and show that the co-amplified loci share the same histological distribution in the tumour and have a similar nuclear distribution within individual nuclei. Regions of the tumours without amplification are easily recognized and tumours with erbB2 and topoll alpha co-amplification can be distinguished from those with erbB2 amplification alone. In addition, FISH was used to show polysomy of chromosome 17 in non-invasive ductal carcinoma in situ of the breast and erbB2 amplification in both the invasive and non-invasive components of a breast cancer biopsy. This report of an interphase cytogenetic analysis of non-invasive breast carcinoma in situ demonstrates the usefulness of FISH for the genetic study of breast cancer progression.