Background <p>Regular screening mammography can detect breast cancer at an earlier stage, leading to better outcomes, including significantly improved survival rates and quality of life. However, the association between delayed mammography and the likelihood of advanced-stage breast cancer remains unclear.</p> Methods <p>This population-based case-case study used Taiwan’s National Health Insurance Research Database linked to the Cancer Registry to examine the association between delayed mammography and advanced-stage breast cancer. Subjects with stages 0–II breast cancer were classified as early-stage, while those with stages III–IV were classified as advanced-stage breast cancer. The main exposure was time since last screening mammogram prior to cancer diagnosis. Multivariable logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (CIs) for advanced cancer, adjusting for demographic and clinical factors.</p> Results <p>11,672 were diagnosed with early-stage breast cancer, and 2,220 with advanced-stage breast cancer. Compared with patients who had undergone mammography screening, those who had never undergone screening had higher odds of being diagnosed with advanced-stage breast cancer (aOR = 1.94, 95% CI = 1.73–2.16). Compared with women with more recent mammography, those with screening intervals of more than two years (aOR = 1.42, 95% CI = 1.18–1.70) and more than four years (aOR = 1.38, 95% CI = 1.13–1.68) had higher odds of advanced-stage breast cancer. Age-specific analyses showed that delaying mammography over four years was associated with higher odds of advanced-stage breast cancer in women aged 40–69, those 70 and older, and in low-income groups.</p> Conclusions <p>Delayed mammography screening is associated with higher odds of advanced-stage breast cancer at diagnosis, particularly among women aged 40 and older and those in low-income groups.</p>