BACKGROUND Fifty-one cases of de novo fibroadenoma in women aged 35 years and older were found during an analysis of 117,729 visits to the Wesley Breast Clinic from 1990 to 1996. METHODS The clinical, mammographic and ultrasound diagnosis of fibroadenoma was confirmed by either fine needle aspiration cytology or histology of an open biopsy specimen. In all cases there was a well-documented previous visit available for review, at which there was no clinical or radiological evidence of the fibroadenoma. RESULTS Thirty-seven of the de novo fibroadenomas were palpable, the remainder satisfying strict mammographic and/or ultrasound criteria. Four of the new fibroadenomas were in women aged 50-52. CONCLUSIONS This study provides information about the natural history of fibroadenomas, confirming that they can appear for the first time in middle-aged women. This has important clinical implications, since new lesions appearing in women over 35 have tended to be automatically categorized as suspicious of carcinoma. However, a multidisciplinary approach involving clinical examination, mammography, ultrasound, and fine needle aspiration cytology or core biopsy can result in a confident diagnosis of fibroadenoma. This will allow some women with new lesions to be managed conservatively rather than by open biopsy.