The present study was designed to assess the performance of fine needle aspiration cytology (FNAC) and core-needle biopsy (CNB) of breast lesions when these procedures are performed under sonographic guidance. The results obtained in 1142 FNAC procedures and 180 CNB procedures were analysed. The study took place in a University Hospital and a private practice clinic. The patients eligible for this study were a series of women, in whom at least one hypoechoic, limited mass was found at breast ultrasonography. Cystic masses were excluded from the study. Each individual mass was investigated using either FNAC or CNB under sonographic guidance. Accuracy characteristics to suspect or diagnose malignant and pre-malignant breast lesions, such as sensitivity and specificity, were calculated. The cytological results were classified into four categories according to standard criteria: benign; atypical and/or suspicious for cancer (hyperplasia with atypia); malignant; and unsatisfactory for diagnosis specimen. Tissue specimens were classified according to the W.H.O. The 1142 lesions submitted to FNAC included 66 invasive carcinomas, 4 intraductal carcinomas, and 4 atypical hyperplasias. FNAC led to 6 false-negative examinations, equally distributed between small (volume < or = 1 cm3), and larger lesions, and 1 false-positive examination. There were 12.9% (147/1142) inadequate smears. Only 3 inadequate samples were obtained in the presence of a discrete pathologic lesion (3/363, or 0.1%). All 3 corresponded to invasive carcinomas. The majority of inadequate samples (144/147, or 98.0%) were obtained in the normal/dystrophic group. Overall, the sensitivity is 92.1%, and the specificity is 84.8%. The 180 lesions submitted to CNB included 31 invasive carcinoma, 5 intraductal carcinomas, and 17 atypical hyperplasias. CNB, in this series, had an accuracy rate of 100%. In conclusion, US guidance increases the accuracy of breast tissue sampling procedures. This is of particular importance as the number of suspicious images to be investigated steadily increases, as the result of mass screening.