Background: Patients with breast cancer tend to limit the use of their arm which may result in muscle reduction and misdiagnosis of breast cancer-related lymphoedema (BCRL). Aims: To examine whether patient-reported arm dominance matched the true dominant arm and if the side of surgery had an impact on grip strength and BCRL. Methods: Grip strength was used to determine the patients’ true dominant arm and assess any muscle weakness. A comparison was made between patients who had surgery on their dominant side and their non-dominant side and between patients who had grip strength weakness in their affected side. Results: 31.6% had a higher grip strength in the perceived non-dominant side. There were 39.5% of patients who had weakness in their affected arm. The grip strength weakness of the affected side was shown in 52.9% in who received surgery on reported dominant side, while it was shown in only 28.6% in the patients who received surgery on the non-dominant side. L-Dex® (ImpediMed) was significantly higher in the patients who received surgery on their perceived non-dominant side compared with the patients who received it on their dominant side (p = 0.031). The relative oedema volume in the forearm (p =0.000) and whole arm (p =0.003) was significantly higher in the patients who received surgery on their perceived dominant side compared with the patients who received it on their non-dominant side. There was no diferent in skin induration between patients with weakness and non-weakness. Conclusion: The patients’ reported dominance might be different from true dominance due to treatment side effects or avoidance of using the affected arm. This might affect the diagnosis of BCRL using L-Dex or circumferences of limb.