The DNA nuclear content of 83 ‘pure’ intraductal cancers (DCIS), 30 DCIS with an invasive component, 7 intraductal papillomas, 26 atypical hyperplasias, and 5 ‘hobnail’ lesions was determined with a Cell Analysis Systems Image Analyzer (CAS 200). No consistent quantitative alteration of DNA or nuclear grade assessed by routine microscopy was observed for any ductal change diagnostic of DCIS. Fifty-eight percent of all DCIS were considered to be aneuploid, whereas all papillomas and atypical hyperplasias were diploid with normoproliferative S-phase and good nuclear grade. Use of ploidy, S phase, or nuclear grade for the distinction of DCIS and papillomas and atypical hyperplasia is limited, since their estimates would be comparable in 42%, 46%, and 40% of instances respectively. Poor nuclear grade was significantly more frequent in solid than cribriform types of DCIS. There were no significant differences in any parameters in DCIS with and without comedo necrosis. This represents further evidence against recognition of a unique comedo form of DCIS. DNA histograms of nuclei comprising the ‘hobnail’ lesions were aneuploid. Despite this their biologic nature is uncertain. No consistent differences in DNA content were noted between DCIS with and without an invasive component. This obfuscates insight into the role of DCIS in the development of invasive cancer of the breast.