Background Some experimental and retrospective clinical studies signal an association between certain anaesthetic techniques and tumour metastasis following breast cancer surgery. Neutrophil Extracellular Trapping (NETosis) is an immunological process, whereby neutrophils engulf tumour antigen then degranulate, leaving a serologic marker. NETosis expression among breast cancer patients is associated with an increased risk of metastasis. We investigated the effect of two distinct anaesthetic techniques on the expression of NETosis in women who underwent potentially curative breast cancer surgery. Methods In a parallel‐group, randomised controlled trial, a subset of women (n = 40) undergoing breast cancer resection surgery, who were partaking in a larger trial (NCT00418457), were randomly assigned to receive volatile general anaesthesia (GA) or propofol GA combined with paravertebral regional anaesthesia (PPA) for their surgery. Serum was taken and stored before and 24 hours post‐operatively. NETosis was measured by ELISA using Neutrophil Myeloperoxidase (MPO) and citrullinated histone H3 (H3Cit) biomarkers, which were the co‐primary end points. Results Patient and breast cancer characteristics did not differ significantly between groups. Recurrence occurred in 7.5% patients. GA patients received more opioids and reported higher post‐operative pain than PPA. There was no difference in post‐operative MPO in GA vs PPA (10.5 ± 6.6 vs 11.5 ± 4.7 ng mL−1, P = .60). Regarding CitH3, there was no difference post‐operatively in GA vs PPA (3.6 ± 2.3 vs 4.0 ± 5.9, P = .80). NET expression did not differ before or after anaesthesia and surgery in either group, for either biomarker. Conclusion Anaesthetic technique did not affect NETosis expression in breast cancer patients, indicating that it is not a viable marker of the effect of anaesthetic technique on breast cancer recurrence.