The risk of malignant change in pre-invasive breast disease such as proliferative atypia (PA) or the risk of invasive carcinoma arising from ductal carcinoma in situ (DCIS) remains uncertain in individual women because of the absence of any prognostic criteria. In order to clarify this, the cellular DNA content (ploidy) of 51 screen-detected lesions has been investigated. Cellular DNA measurements were made by static cytometry following Feulgen staining of disaggregated tissue sections and the resulting histograms classified as either diploid or aneuploid. Thirteen cases of PA and twelve of DCIS were compared with twenty-six biopsies showing DCIS with adjacent invasive carcinoma (DCIS + Ca). In the latter group, ploidy of the invasive carcinoma was compared with the associated DCIS in 16 cases. Aneuploid cells were found in approximately 30 per cent of PA and DCIS lesions but in 23 of 26 cases of DCIS + Ca. Of 16 assessable cases of co-existing DCIS and micro-invasive carcinoma both were aneuploid in 11, both diploid in 1, and in 4 cases the DCIS was aneuploid whereas the invasive carcinoma was diploid. These results suggest that aneuploidy may be of value in predicting the most biologically aggressive of these pre-invasive lesions.