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Anticoagulant treatment might enhance the natural defense against tumor cell dissemination caused by diagnostic needle biopsy by counteracting thrombocyte coating of such cells. To clarify whether women using anticoagulant treatment at the time of biopsy have a lower occurrence of lymph node metastasis, we conducted a nationwide Swedish cohort study of 26,528 female incident breast cancer patients in 2006–2011. Point risk ratio (RR) of risk of lymph node metastasis among users of anticoagulant treatment adjusted for age, T-stage, socioeconomic factors, and concomitant medication was RR = 0.94, (95% CI: 0.87–1.03), and lower in younger women (RR = 0.80, 95% CI 0.50–1.29). Although nonsignificant, these associations may underestimate a true negative association since women using anticoagulant treatment are likely to have more concomitant diseases, lead an unhealthier lifestyle, and have lower participation in mammography screening. These findings provide some support for the hypothesis that anticoagulant medications might counteract breast cancer spread caused by needle biopsy.Diagnostic needle biopsy carries a risk of tumor spread, particularly for tumors with unstable genomes. It has been postulated, however, that anticoagulant treatment may reduce cancer cell dissemination via needle biopsy. Here, in a large breast cancer cohort, a nonsignificant decrease in risk of lymph node metastasis was observed among users of anticoagulant treatment. However, factors such as relatively poor baseline health among anticoagulant users may mask a true negative association. Thus, the study may lend some support to the idea that tumor spread might be counteracted by anticoagulants at the time of needle biopsy.