BACKGROUND: The diagnosis of phyllodes tumors of the breast is challenging due to many similarities with common fibroadenomas. PURPOSE: To determine the diagnostic accuracy of core needle biopsy in diagnosing phyllodes tumors and to analyze the ultrasonographic (US) features of phyllodes tumors and fibroadenomas. MATERIAL AND METHODS: From 1999 to 2003, 1010 breast lesions underwent imaging-guided core needle biopsy. Of these, 57 fibroadenomas and 12 phyllodes tumors were removed surgically. The US and needle biopsy results of a total of 64 lesions (52 fibroadenomas and 12 phyllodes tumors) were further analyzed, compared, and correlated with surgical histological results. RESULTS: The median sonographic sizes of the phyllodes tumors and the fibroadenomas were 3.2 cm and 1.6 cm, respectively. At US, 58% of the phyllodes tumors (7/12) were classified as equivocal or suspicious of malignancy and 42% (5/12) as probably benign, while 54% of the fibroadenomas (28/52) were classified as probably benign and 46% (24/52) as equivocal. The sensitivity, specificity, and positive and negative predictive values of core needle biopsy histology regarding tumor phyllodes were 83%, 92%, 71%, and 96%, respectively. CONCLUSION: Imaging-guided core needle biopsy was accurate in differentiating between fibroadenomas and phyllodes tumors. US classification was unreliable due to considerable overlap in the findings. Combined use of US feature analysis and needle biopsy may help to avoid the misinterpretation of phyllodes as fibroadenoma.