PURPOSE: To determine the effect of a selective core biopsy program on the yield at needle-localization biopsy (NLB) of nonpalpable lesions. MATERIALS AND METHODS: Two hundred consecutive core biopsy samples of the breast were evaluated in an ongoing consecutive series of 1,172 NLB samples. RESULTS: Before implementation of the core biopsy program, the yield at NLB improved from 21% at 100 cases to 35% just before the introduction of core biopsy. After implementation, the yield increased gradually to 55% at 200 cases. The yield in masses increased from 21% at 100 cases to 43% just before the initiation of the core biopsy program and then increased dramatically to 72% at 200 cases. The percentage of small lesions detected did not change with implementation: 88% of invasive cancers measured less than 1.5 cm and 60% measured less than 1 cm in the last 100 cases. CONCLUSION: Appropriate selection of cases for core biopsy can more than double the yield of cancer in NLB samples without a decrease in the percentage of small cancers detected.