The Essential Number of Ultrasound(US)- Guided Core Needle Biopsy in Diagnosis of Breast Cancer Dr. Zahraa Abd Al-Kareem Mahmmod and Dr. Kaswer Mousa Al-Tariahi Background: Breast cancer is the one of most common cancer among women worldwide, both in developed and developing countries and represent the second most common cause of cancer death in women age between 40-59.so the accurate preoperative tissue diagnosis is essential before definitive surgical treatment for breast cancer. Objective: To detected the essential number of US- guided breast core biopsy in the diagnosis of breast cancer according to the percentage of core involvement by the lesion. Method: 155 specimens from archive of AL sadder Teaching Hospital was collected,44 specimens were excluded from the study because they are fragmented and other insufficient data,166 specimens then divided to the benign, atypical and malignant. The malignant lesion then reviews to determine the percentage of each core involved by the malignant lesion. Result: 70(66.03%) lesions were malignant, five (4.7%) were atypicaland31(29.2%) were benign. (53%) of two cores malignant lesion have (26_50%) involve of the cores and the percentage increase with increase the number of the cores. Conclusion: US-guided core needle biopsy is more effective technique in diagnosis of breast lesion and find that two cores having a good percentage for reach to the diagnosis.