BACKGROUND: After introduction of sentinel node biopsy (SNB) in patients with breast cancer a higher proportion of micrometastases and isolated tumor cells are being detected. Prognostic impact and clinical relevance of this minimal nodal involvement is under debate and substantial variation in the use of axillary surgery and/or systemic adjuvant treatment could be expected. METHODS: Data from the population-based Eindhoven Cancer Registry were used on all (n = 9038) women who underwent SNB for invasive breast cancer from 1996 to 2008 and medical files were studied to determine the role of minimal nodal involvement in the decision to use adjuvant systemic treatment. RESULTS: Forty-five percent of 172 patients with isolated tumor cells and 76% of 605 patients with micrometastases received adjuvant systemic treatment. Thirty-five of 59 patients with isolated tumor cells and 153 of 193 patients with micrometastases received systemic therapy based on primary tumor characteristics. The remainder probably received adjuvant therapy based on presence of minimal nodal involvement. Thirty-seven percent of the patients with isolated tumor cells underwent an axillary lymph node dissection compared to 75% when micrometastases were present. Multivariate analyses showed a significantly higher chance of receiving systemic treatment when isolated tumor cells (OR 1.5 (95% CI, 1.05-2.15)) or micrometastases (OR 10.7 (95% CI, 8.56-13.27)) were present, compared to a negative lymph node status. CONCLUSION: The debate on necessity of performing completion ALND and administration of systemic therapy in patients with minimal nodal involvement is reflected by the treatment patterns observed in our population-based study. SYNOPSIS: Describing time-trends and predictors of axillary and systemic treatment of patients with breast cancer and micrometastases or isolated tumor cells in their sentinel lymph node(s).