Background: Screening mammography has limited sensitivity for detecting breast cancer in dense breast tissue. This study estimates the maximal number of breast cancers undetected by mammography in individuals with dense breast tissue participating in screening mammography in the United States. Methods: Published data on supplemental screening incremental cancer detection rates (ICDRs), dense breast tissue prevalence, and total annual screening mammography exams in the United States are utilized for study estimates. Results: Assuming an ICDR of 16 cancers beyond mammography per 1,000 individuals with dense breast tissue, 38.8 million mammograms in the U.S. in 2021, and a prevalence of dense breast tissue of 43%, the number of cancers undetected by mammography in individuals with dense breast tissue participating in screening in the U.S. is estimated at 267,000. Conclusion: A large number of undiagnosed breast cancers in the population of individuals with dense breast tissue participating in screening mammography is estimated.