We recently changed from using fine needle aspiration cytology to using core biopsy exclusively in the assessment of screen detected abnormalities. Two hundred and two biopsies (1% of women screened) were performed. Surgical histological confirmation was obtained in 111 patients (101 malignant and 10 benign). The remaining patients were either returned to standard 3-yearly screening or early repeat screening after 1 year. Analysis of the results was performed in accordance with the standards specified in the National Health Service Breast Screening Programme (NHSBSP) Publication Number 22. Absolute sensitivity was 89.3%, complete sensitivity was 93.2%, specificity (including patients undergoing both surgical excision and follow-up) was 88.7%. The predictive value of a positive (malignant) core biopsy result was 100%. The false negative rate was 3.9%. Twelve (5.9%) biopsies were classified inadequate for diagnosis. Core biopsy is a safe and accurate way of assessing screen detected abnormalities and can be used as a substitute for fine needle aspiration cytology with results that exceed the National Health Service Breast Screening Programme target standards, even in the learning phase.