Breast needle core biopsy (NCB) is now a standard diagnostic procedure in the triple assessment of screen detected breast lesions. However, unlike fine needle aspiration (FNA) cytology, information on the miss rate including false-negative diagnoses (FN) of malignancy (benign 'B2' or normal 'B1' NCB with a malignant outcome) is limited. Methods: A large series of NCBs (121, 742) performed over an 8-year period has been studied to assess the frequency and causes of missing a malignant diagnosis on NCB and to evaluate their impact on patients' management in the screening service. Results: During the period of this study, 50,691 were diagnosed as B2 and 9599 were diagnosed as B1. Of those, 779 B2 and 919 B1 were diagnosed as malignant on the subsequent surgical specimens, respectively, giving a FN rate of 3.0%. However when year of diagnosis was taken into consideration, we found that during the period 1999-2001, the FN rate for B2 was 2.7% while the miss rate for B1 was 4.0%. This showed marked improvement over time to reach a figure of 0.5% and 0.5% for B2 and B1, respectively, during the period 2005-2007. On detailed review of cases from a single screening region diagnosed during the last 3 years (2005-2008), 14 cases (0.17% of all NCBs) with malignant surgery were diagnosed as B2 (seven cases; FN rate 0.19%) and B1 (seven cases; B1 biopsy rate from cancer 0.19%). In these cases, NCB was unsatisfactory, there was a discrepancy between radiological abnormalities and histological findings with recommendation for excision or suspicious/malignant cytological diagnosis on concurrent FNA material. Therefore, our results indicate that the malignancy miss rate on NCB is rare and FN NCB diagnoses had no impact on patient management.