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Introduction: Most women who have screening mammography and undergo subsequent open biopsy following an indeterminate core biopsy result are eventually found to have benign disease. However, a significant number have malignant disease and the rate of malignancy in such cases may be influenced by various factors. This study examined the effect of the type of screening round (prevalent or incident) on the likelihood of breast cancer being present., Methods: A total of 199 women who had NHS breast screening mammograms and subsequent indeterminate (B3) core biopsy results followed by excision biopsy over an 11-year period in a single breast screening unit were reviewed., Results: The rate of malignancy following excision of a lesion graded as B3 on core biopsy was 21% for women in the prevalent screening round compared to 33% in subsequent rounds (Fisher's exact test, p=0.038)., Conclusions: The incidence of malignancy associated with a B3 core biopsy result appears to be related to the screening round in which the lesion is detected, being approximately 50% higher in the subsequent incident rounds compared to the initial prevalent round. This finding may be useful in formulating management plans for women who have an indeterminate biopsy result.