The present paper describes our experience of 47 cases of atypical ductal hyperplasia (ADH) diagnosed at vacuum-assisted biopsy. From June 1999 to December 2003, 47 consecutive diagnoses of non-palpable ADH of the breast were made by 11-gauge vacuum-assisted biopsy (Mammotome). Of these, 17 were subjected to surgical excision and 11 underwent a second Mammotome at the site of the previous vacuum-assisted biopsy. Diagnostic underestimation occurred in only two cases, with a surgical diagnosis of ductal carcinoma in situ. In both patients, aged between 46 and 55 years, the radiological images showed microcalcifications of >20 mm, and the lesions were not completely removed by Mammotome. Despite the obvious limitations of the present study, it can be concluded that the probability of underestimating ADH diagnosis by Mammotome appears to be related to the radiological features of the lesion (>20 mm) and to the adequacy of specimens.