Fine-needle aspiration (FNA) is a valuable technique to use in the evaluation of breast lesions; however, inadequate and discrepant diagnoses do occur. To identify the source and nature of inaccuracies related to the method we studied 39 cases in which FNA posed diagnostic problems. These problems could be attributed to sampling errors (71.8%), to the criteria of adequacy we use at our institution (25.6%), and to interpretation (2.6%). The nature of the breast lesion (68%) was the most common cause of inadequate sampling, followed by the experience of the aspirator (32%).