The objective of this study was to develop a standard diagnostic evaluation with 16G core-needle biopsy for breast lesions. We evaluated the ideal number of fragments using standard ultrasound criteria based on the previous results obtained on the size of the lesions.A prospective study was carried out, from may 2004 to September 2005, in 79 patients with lesions that included categories 2,3,4 and 5, according to BIRADS -USTM using 16G core biopsy needle. We took up 5 pieces numbered and placed individually in vials with 10% formalin.In 84 biopsies performed there were 81 conclusive diagnoses (96%), with 43 malignant results (51%) and 38 benign results (45%). The effectiveness of the core biopsy increased with the sample numbers: one sample is 95.24% accurate; two samples reach 96.93%; three samples reach 98.8%; four samples reach 98.81%, and finally, five samples reach 100% accuracy.the resection of three fragments was sufficient for a satisfactory result.