Objective: The authors evaluated the differences between stereotactic core needle biopsy (SCNBx) and needle localization surgical biopsy (NLBx) in cost and treatment course for patients with mammographically detected breast cancer. Summary Background Data: Stereotactic core needle breast biopsy is a reproducible and reliable alternative to surgical biopsy for histologic diagnosis of mammographic lesions. Methods: Records from 52 consecutive patients with invasive breast cancer diagnosed by SCNBx (n = 21) or NLBx (n = 31) over 2 years were reviewed Episode-of-care costs were extracted from the Barnes Hospital billing system database. Results: At the time of excision, surgical margins were statistically more frequently positive in patients treated with NLBx (55%) than patients treated with SCNBx (0%, p < 00001). Furthermore, patients in the NLBx group undergoing breast conservation surgery required re-excision more frequently (74%) than those in the SCNBx group (0%, p = 0.001). There were no complications in either group after the diagnostic procedure. All SCNBx results were correct in the diagnosis of invasive breast cancer. The median cost of SCNBx was approximately $1000 less than the median cost of NLBx. This cost difference was carried through the definitive procedure, whether it was breast conservation or masteotomy. Conclusions: This study shows the advantage of SCNBx to diagnose breast cancer and definitive operative care at a single procedure. The preoperative diagnosis of breast cancer eliminated positive operative margins and procedures to re excise breast tissue. The use of SCNBx also saved approximately $1000 per patient compared with the use of NLBx. Our data suggest that SCNBx is the diagnostic procedure of choice for mammographically detected cancers.