Background: Ductal carcinoma in situ (DCIS) in the breast that is diagnosed by biopsy implies a risk of upstaging to invasive carcinoma (IC) on final pathology. These patients require a sentinel lymph node biopsy (SLNB) for axillary staging. A two-stage procedure is not always feasible and precise selection of patients who should be offered SLNB is crucial. The aims were: to determine the rate of upstaging, and use of redundant and required SLNB in women with a preoperative diagnosis of DCIS; and to identify patient and tumour characteristics that increase the risk of upstaging. Methods: Patients with DCIS treated between 2008 and 2016 were identified using Orbit operation planning system software, and those suitable for the study were selected based on review of the medical records. Upstaging rates and proportions of redundant and required SLNBs were calculated. Associations between clinicopathological characteristics and upstaging were analysed using univariable and multivariable logistic regression analyses. Results: Of 1368 patients initially identified, 975 women with a preoperative diagnosis of DCIS were included in the study. Tumours in 246 of these patients (25·2 per cent) were upstaged to IC. Redundant SLNB was performed in 392 of 975 women (40·2 per cent). Forty-four patients (4·5 per cent) with a final diagnosis of IC were not offered SLNB and thus potentially undertreated. In adjusted analysis, DCIS size, palpability and mass formation identified by breast imaging were associated with increased risk of upstaging. The Van Nuys classification was not associated with upstaging. Conclusion: Most patients with IC on final pathology underwent SLNB, but a considerable number of patients with DCIS had a redundant SLNB. Lesion size, palpability and mass formation, but not Van Nuys classification group, are suggested risk factors for upstaging.