Background Breast cancer patients undergoing mastectomy are not well represented in studies evaluating the omission of completion axillary lymph node dissection (cALND) due to sentinel lymph node (SLN) metastases.Methods The prospective multicentre SENOMIC cohort trial included patients with breast cancer and SLN micrometastases between October 2013 and December 2022. Completion ALND was omitted and nodal radiotherapy discouraged. Patients undergoing mastectomy were selected for the present analysis. The primary endpoint was event-free survival. Secondary endpoints were the isolated axillary recurrence rate (ARR) and cancer-specific and overall survival.Results In the SENOMIC trial, 455 of 882 included patients underwent mastectomy, 407 of whom did not receive nodal radiotherapy. The median follow-up time was 53 (range 5-109) months. The estimated 5-year event-free, cancer-specific, and overall survival were 86.8%, 97.0% and 91.6%, respectively. Isolated axillary recurrence was found in 14 patients (3.1%).Conclusion Event-free survival after omission of cALND in breast cancer patients with SLN micrometastases undergoing mastectomy was excellent, despite a higher ARR than previously reported. Long-term follow-up is important to validate the safety of this approach, but omission of cALND should be regarded as routine care. This is a 53-month follow-up of 455 breast cancer patients with sentinel lymph node micrometastases undergoing mastectomy without completion axillary lymph node dissection. Event-free, cancer-specific, and overall survival were excellent. The isolated axillary recurrence rate was higher than in most previous studies.