We have analysed DNA from 183 primary breast cancers for amplification or rearrangement of a number of cellular proto-oncogenes, focusing primarily on a cluster of markers on the long arm of chromosome 11. Two of these oncogenes, INT2 and HST1, both of which encode members of the fibroblast growth factor family, are implicated in the generation of virally induced mammary tumours in mice. Here we confirm earlier reports that the q13 region of chromosome 11, in which these genes are tandemly linked, is modestly amplified in approximately 15% of primary human breast cancers. This amplification is confined, with one exception, to cases in which the oestrogen receptor (ER) levels are in excess of 20 fmol/mg protein (P = 0.001). However, DNA amplification does not usually result in detectable expression of either the INT2 or HST1 gene. The data imply that some other gene in the vicinity must contribute to the development of a subset of ER-positive tumours and that assessing the amplification of this region of DNA may be of value in defining a separate category of ER-positive tumour.