The reliability of stereotactic core biopsy in the diagnosis of malignant and premalignant breast lesions was assessed in comparison to excision biopsy in patients with non-palpable suspicious breast lesions detected in a mammography breast screening programme. Fifty-two cases of malignancy and nine of atypical ductal hyperplasia were diagnosed on the programme during the two year period July 1993 to June 1995; two patients did not have excision biopsy. Stereotactic core biopsies and representative sections from 59 excision specimens from the same patients were assessed "blind' by one pathologist. All 51 cancers diagnosed on stereotactic core biopsy were confirmed to be malignant on excision biopsy. There was 96% concordance between stereotactic core biopsy and excision biopsy for the diagnosis of invasive or in situ cancer, and 78% concordance for the type of cancer. The stereotactic core biopsy and excision biopsy diagnoses were: invasive ductal carcinomas (39 on stereotactic core biopsy vs. 33 on excision biopsy), mucinous carcinomas (1 vs. 2), invasive lobular carcinomas (3 vs, 8), and in situ carcinomas (8 vs. 8), two of which had invasive cancer present only in the stereotactic core biopsy. Of the nine cases of atypical ductal hyperplasia diagnosed on stereotactic core biopsy, eight had an excision biopsy, six showed low nuclear grade in situ or invasive cancer, one had a 3 mm focus of high grade invasive ductal cancer and one was atypical ductal hyperplasia. In the invasive ductal carcinoma group stereotactic core biopsy underestimated tumour grade: In nine cases (31%) the cancer at excision was of a higher grade. Stereotactic core biopsy is a reliable alternative to excision biopsy in the diagnosis of breast cancer, however, stereotactic core biopsy may underestimate tumour grade in invasive ductal carcinoma and may not differentiate between invasive ductal carcinoma and lobular carcinoma. It is recommended that the diagnosis of atypical ductal hyperplasia on stereotactic core biopsy be followed by excision biopsy, as stereotactic core biopsy underestimates the presence of cancer in this group.