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Introduction of sentinel node biopsy (SNB) as an axillary staging procedure in breast cancer patients could have led to upstaging as well as downstaging of their disease. Intensified pathological processing could have led to upstaging due to presence of micrometastases, whereas the described false negative rate of 5% could be a cause of downstaging. We investigated whether or not the introduction of the sentinel node procedure has changed the incidence of axillary nodal micrometastases and induced stage migration on a population based level.Data from the population based Eindhoven Cancer Registry were used on all (n, =, 17100) women diagnosed with invasive breast cancer in the South-East region of The Netherlands in the period 1994–2005.The percentage of patients staged with SNB increased from 0% in 1994 to 62% in 2005. The percentage of patients with micrometastases increased from 1.0% in 1994 to 4.3% in 2005 (P, <, 0.0001), whereas no significant increase was observed of the proportion of patients with positive axillary lymph nodes. After adjustment for tumor size, age at diagnosis and histology the probability of having a positive axillary lymph node status was 10% higher in period 2003–2005, as compared to period 1994–1996. Furthermore, women diagnosed in the period 2003–2005 had a 3.5, times higher risk of having micrometastases compared to women diagnosed in the period 1994–1996 (i.e., before the introduction of SNB).The introduction of SNB in the Southeast region of The Netherlands has led to stage migration, as is reflected by the small but significant increase of the proportion of patients with positive axillary lymph nodes after adjustment for tumor size and age.