The UK breast screening Surgical Quality Assurance guidelines suggest a target for pre-operative diagnosis of screen detected cancer of over 70%. Core biopsy was introduced in our breast screening assessment clinics in April 1994 and this study reports the impact of introducing core biopsy on the pre-operative diagnosis rate. Between April 1994 and March 1995, 100 cancers were detected. Results of fine needle aspiration cytology and core biopsies were studied to assess the contribution of each to the pre-operative diagnosis rate of cancer and comparison made with results from the previous 2 years. After introducing core biopsy pre-operative diagnosis rates rose from 72% to 90% (P < 0.0002) leading to a 64% reduction in diagnostic biopsies for screen detected cancer. Malignant results were obtained in 61% of first FNA (similar to previous years) and 74% of first core biopsies (combined 87%). Repeat FNA/core diagnosed three further cancers increasing the final pre-operative diagnosis rate to 90%. Final pathological examination after surgical excision demonstrated an increase in the preoperative detection of DCIS from 39% to 82% (P < 0.01) and invasive disease from 80% to 92% (P < 0.02). The introduction of core biopsy has significantly improved our management of screen detected breast cancer.