The mammograms of 86 women under the age of 50 years with mammographically detected cancers were reviewed. Sixty-three of these cancers (73%) were found at the periphery of the breast as defined by a zone 1 cm wide beneath the subcutaneous fat or anterior to the retromammary fat. This finding is likely due to the geometric configuration of the breast. If the breast is hemispheric, then more than 50% of the volume of breast tissue lies within this zone in a large percentage of women. Furthermore, the contrasting adjacent fat likely improves the ability to perceive an abnormality. Interaction of the terminal ducts with chemicals contained within or produced by the fat is a much less likely reason for the high percentage of cancers in this zone. Radiologists should be aware of this phenomenon and pay particular attention to this zone.