The Feulgen DNA content and the nuclear measurements of four groups of intraductal proliferations of the breast (hyperplasia, atypical hyperplasia, well-differentiated carcinoma without cytologic atypia and intraductal carcinoma with cytologic atypia) were compared. Intraductal carcinoma with atypia was the only group distinct from the others on the basis of DNA content, nuclear area and perimeter. Although the other groups were separable from intraductal carcinoma with atypia, they could not be reliably distinguished from each other by any combination of measurements. At best, 69% of well-differentiated intraductal carcinomas could be distinguished from atypical hyperplasias using a combination of DNA content and nuclear perimeter measurements. Thus, the difficult distinction of atypical hyperplasia from well-differentiated intraductal carcinoma by light microscopy was not aided by DNA analysis or by nuclear measurements.