Introduction: The clinical importance of internal mammary (IM) lymph node (LN) metastases in breast cancer remains unclear. The aim of this study was to determine the clinical value of opportunistic IMLN sampling at the time of free flap breast reconstruction using the IM recipient vessels and whether it affected the adjuvant treatment given. Methods: A prospective study was conducted of IMLN exploration performed by a single surgeon as part of free flap breast reconstruction using IM recipient vessels. Patients where IMLNs were positive for metastatic disease were reviewed with the breast cancer multidisciplinary team for changes in therapy. Results: 122 patients met the inclusion criteria, 111 of whom had immediate reconstructions. 63 patients had IMLNs excised, and of these 13 were positive for metastatic disease, all in immediate breast reconstructions. The adjuvant treatment given was changed in 1 patient with no axillary LN disease as a result of finding a positive IMLN. Positive IMLNs were significantly associated with larger tumours and axillary metastases, but not tumour location. All patients with positive IMLNs were alive at last follow-up with no local or distant recurrences, with mean disease-free survival of 20.5 months (5-56 months). Conclusion: Incidental IMLNs positive for metastatic disease were found in 13 of 122 free flap breast reconstructions and resulted in a change in adjuvant treatment in one patient. IMLNs found incidentally during recipient IM vessel exposure for free flap breast reconstruction therefore may upstage a patient's disease and influence the adjuvant treatment for their breast cancer and should therefore be submitted for pathological assessment.