Stereotactic core biopsy was performed on 200 women for 206 mammographically suspicious non-palpable lesions detected over a period of 2 years as part of the Australian national programme for early detection of breast cancer. This study aimed to assess the reliability of stereotactic core biopsy in this context and to develop a protocol for the evaluation of stereotactic core biopsy in mammographically detected non-palpable breast lesions. Fifty-one of 52 malignant lesions found by stereotactic core biopsy were confirmed by excision biopsy (one women declined excision). Nine (4.5%) women had atypical ductal hyperplasia on stereotactic core biopsy; at excision, six were low grade carcinomas (in situ or invasive carcinomas), one was a 3 mm focus of grade 3 invasive duct carcinoma, one was atypical ductal hyperplasia, and one patient refused excision biopsy. In 29 (14.5%) women the histology of the stereotactic core biopsy was considered not to correlate with the radiological abnormality, and excision biopsy was advised: in four of these women carcinomas were found. One hundred and ten (55%) women had 116 benign lesions on stereotactic core biopsy: on follow-up, one of these patients has been found to have a carcinoma. Core biopsy number and sequence were analysed demonstrating that no particular biopsy was more diagnostic than any other, and that the diagnostic yield of three cores was statistically equal to that of five cores. The procedure was well-tolerated and there were few complications. Thus, stereotactic core biopsy is an accurate and safe method for diagnosis of mammographically detected non-palpable breast lesions, and we believe it is the diagnostic technique of choice in breast cancer screening programmes. However, a stereotactic core biopsy diagnosis of atypical ductal hyperplasia requires excision biopsy since a diagnosis of low grade intraduct carcinoma cannot be excluded. Furthermore, if tissue obtained by stereotactic core biopsy does not correlate with the mammographic abnormality, excision biopsy should be performed.