Hematoma resulting from attempted aspiration of a palpable breast mass can cause incorrect mammographic interpretation by rendering irregular and indistinct the otherwise smooth and sharply defined margins characteristic of a benign lesion. In this study of recently aspirated breast masses, we found 17 benign lesions that demonstrated poorly defined, irregular margins on mammograms that suggested malignancy. All these false-positive interpretations occurred when aspiration preceded mammography by less than 2 weeks (17 of 47, 36 percent); no such diagnostic error occurred in the 31 cases when mammography was delayed for 2 weeks or more after aspiration. An appreciation of the frequency and natural history of postaspiration hematoma formation should either encourage physicians to request mammography before carrying out invasive procedures, or alternatively, to defer mammography until 2 weeks after aspiration, since mammography then should more clearly portray the benign characteristics of truly benign masses, thereby possibly obviating biopsy.