The objective of our study was to evaluate organized breast cancer screening implementation by measuring the association between screening program enrollment and late-stage disease. Our setting was a health plan using mailed mammography reminders to women ages > or = 40. We conducted yearly cross-sectional summaries of mammography experience and late-stage (regional or distant Surveillance Epidemiology and End Results Reporting (SEER) stage) breast cancer occurrence for all of the health-plan women ages > or = 40 (1986-1998). We estimated the odds of late-stage breast cancer among health-plan and surrounding community women because it was too early to compare changes in mortality. We also estimated the odds of late-stage disease (1995-1998) associated with program enrollment and mammography screening among health-plan women. We found that mammography-within-two-years increased within the health plan from 25.9% to 51.2% among women ages 40-49 and from 32.9% to 74.7% among women ages> or = 50. Health-plan late-stage rates were lower than those in the surrounding community [ages 40-49: odds ratio (OR), 0.87; 95% confidence interval (CI), 0.77-0.99; ages 50-79: OR, 0.86; 95% CI, 0.80-0.92] and declined parallel to the community. Among health-plan cancer cases, women ages > or = 43 who were enrolled in the screening program and who had at least one program mammogram were less likely to have late-stage disease compared with the women not enrolled in the program (OR, 0.31; 95% CI, 0.16-0.61) but the odds of late-stage was also reduced among program-enrolled women not receiving program mammograms (OR, 0.45; 95% CI, 0.21-0.95). We concluded that enrollment in organized screening is associated with increased likelihood of mammography and reduced odds of late-stage breast cancer. Addressing the concerns of un-enrolled women and those without mammograms offers an opportunity for further late-stage disease reduction.