From January 1981 to December 1995, 8,421 patients having a mammographic opacity corresponding to breast cancer were examined. The opacities were divided into two groups: zones of increased density (ZID), (1,449 patients), and a reference group comprising all other types of opacities (6,972 patients). Mammograms were reviewed retrospectively, as well as clinical, histological and biological parameters in both groups. Diagnosis of ZID was often difficult, sometime's guided by the presence of microcalcifications, which were more frequent. Sonography and direct mammographic magnification proved to be particularly helpful for a correct diagnosis. Most prognostic factors were poorer in ZID as compared to the reference group. The 5 year survival rate was 78% (84% for the reference group). Breast cancer that appears on mammograms as a ZID is often more difficult to diagnose and factors of poor prognosis are more often observed than in breast cancer with other types of opacities. To improve their detection sonography and direct mammographic magnification can be very helpful.