Our goal was to determine differences in outcome measures between women undergoing annual versus biennial screening mammography.A retrospective review of prospectively collected data on 24,211 consecutive screening mammography examinations was performed in women aged 40-79 years, all of whom had undergone previous normal screening mammography. Annual screening and biennial screening were defined as examinations performed 10-14 months and 22-26 months, respectively, after previous normal screening mammography. The rates of recall, biopsy, cancer detection, and interval cancer for annual and biennial screening cohorts were calculated, as were tumor size, lymph node status, and stage of invasive cancer. Interval cancer cases were identified by linkage with a regional tumor registry.Of the 4306 biennially screened women, 160 were recalled (3.7%), 45 were biopsied (1.0%), and cancer was detected in 19 (0.44%). Of the 19,905 annually screened women, 518 were recalled (2.6%), 150 were biopsied (0.75%), and cance...