The messenger RNAs for the angiogenic acidic and basic fibroblast growth factors are expressed at a significantly higher level in samples of human benign neoplastic and hyperplastic tissue than in samples from breast cancers. However, approximately one in four malignant breast cancer samples contain basic fibroblast growth factor mRNA at the same level as in the benign lesions when basic fibroblast growth factor mRNA levels are corrected with respect to levels of expression of glyceraldehyde-3-phosphate dehydrogenase mRNA. A similar proportion of human malignant breast cancer cell lines express a high level of basic fibroblast growth factor mRNA. The results suggest that some malignant breast cancers and their constitutive carcinoma cells express abundant levels of basic fibroblast growth factor mRNA. The resultant production of basic fibroblast growth factor by breast cancer cells within some tumours may contribute to their development.