SUMMARY: We performed a retrospective study, based on ultrasounds performed in the Department of Mastology of the Central Hospital of the Instituto Prevision Social, in the period from September 2010 to December 2011. A total of 1031 ultrasounds were evaluated, the study included 184 cases with solid nodular formations, the inclusion criteria were that nodules were not palpable, with no larger than 1 cm in any diameter, representing a total of 48 patients who were classified according to their size, all were evaluated histologically by obtaining the material by the method of Trucut type core biopsy, caliber of 16G and 18G, 5 (five) complete samples were taken in each case, all under ultrasound guidance, obtaining initial diagnosis of cancer in T1b tumors in 8 cases, with an average size of 7. 2 mm. , with histological diagnosis of infiltrating ductal carcinoma, in situ ductal carcinoma and infiltrating lobular carcinoma. MATERIAL AND METHODS: Retrospective study, based on ultrasounds performed in the Department of Mastology of the Central Hospital of the Instituto Prevision Social, in the period from September 2010 to December 2011. A total of 1031 ultrasounds were evaluated, the study included 184 cases with solid nodular formations, the inclusion criteria were that nodules were not palpable, with no larger than 1 cm in any diameter, representing a total of 48 patients, obtaining the material by the method of Trucut type core biopsy, caliber of 16G and 18G, 5 (five) complete samples were taken in each case. RESULTS: of the 48 breast nodules smaller than 1 cm, 40 were benign and 8 malignant nodules. In this group of non-palpable breast nodules under 1 cm, the diagnoses of breast cancer varied in size between 9. 7 mm and 5. 4 mm, with an average of 7. 2 mm. The pathologic diagnosis of benign lesions were 32 fibroadenomas, followed by 5 cases of microcalcifications, 3 cases of lymphoma DISCUSSION: The results of the fragments obtained by core biopsies in solid nodules no larger than 1 cm, in this work is the same as those obtained through the same methods mentioned in the literature. Therefore nodules smaller than 1 cm can be treated safely with core biopsy CONCLUSION: in the group of breast nodules under 1 cm, the diagnosis of malignancy guided needle was 100%, all under ultrasound guidance, obtaining an initial diagnosis of cancer in T1b tumors in 8 cases, an average size of 7. 2 mm. , with histological diagnosis of infiltrating ductal carcinoma, ductal carcinoma in situ and lobular carcinoma.