Background: Skin sparing mastectomy (SSM) and nipple sparing mastectomy (NSM) and immediate reconstruction by an implant are widely used in patients with breast cancer and in high risk patients for prophylaxis. Surgical procedure is guided by oncological aspects on the one hand – meaning cutaneous/subcutaneous envelope as thin as possible – and by cosmetic aspects on the other hand – meaning envelope as thick as possible. Pre-operative MRI might help the surgeon to choose the right thickness of this envelope. In this retrospective analysis we investigated the presence of residual breast tissue and the thickness of the envelope after SSM and NSM. Methods: Patients treated with SSM or NSM for prophylaxis or after breast cancer at a single university hospital, who had at least one post-operative MRI, were included. MRI’s were retrospectively analysed by two independent radiologists. Analyses included (a) the detection of residual breast tissue (yes/no) and (b) the maximum thickness of the envelope directly above the implant. Results: 39 patients of which 19 had bilateral SSM/NSM were included. 14 patients with BRCA 1/2 mutation had prophylactic bilateral SSM/NSM. Residual breast tissue could be detected in all 58 reconstructed breasts. The medium thickness of the envelope was 11.2mm (1-35mm). In patients with bilateral SSM/NSM, a strong correlation between the thickness of left and right breast envelope could be observed (r = 0.79, p < 0.05). Conclusions: Thickness of the envelope of the reconstructed breast does not appear to be the major determining factor of residual breast tissue.