Purpose: To evaluate whether platform switching could preserve marginal bone around implants up to 6 months after loading. Materials and methods: 15 patients were selected for a randomised controlled trial. Each patient received one customised wide body implant, with the external hex connection located eccentrically, allowing an extra 1 mm switch on one side. The hex was positioned at random at the mesial or distal side and the implant was loaded after 6 months of non-submerged healing. Patients were examined at 3, 6 and 12 months after surgery, during which a radiograph was taken to evaluate bone levels. At 12 months, the mucosal thickness was measured using a perio-probe. Results: All implants survived and the mean overall bone loss, calculated from both the switched and non-switched side, was 0.39 mm (SD 0.33, range 0.00-1.45), 0.85 mm (SD 0.59, range 0.10-2.50) and 0.80 mm (SD 0.46, 0.26-1.89) after 3, 6 and 12 months, respectively. The bone loss continued up to 6 months but stabilised thereafter (P = 0.615). Significantly more bone loss was observed at the non-switched side compared to the switched side at 3 months (0.51 mm versus 0.28 mm, P = 0.019), 6 months (1.05 mm versus 0.64 mm, P = 0.002) and 12 months (0.94 mm versus 0.66 mm, P = 0.002). The mean mucosal thickness was 4.22 mm (SD 1.45; range 1.50-7.00), and was not significantly different between the switched and non-switched sides (P = 0.882). However, using a post-hoc analysis with the mean thickness as a threshold, the mean bone loss was only significantly different between switched and non-switched sides when the mucosa was thicker than 4.22 mm (P = 0.036). Conclusions: The outcome of this randomised trial is in accordance with earlier studies suggesting that that platform switching decreases bone loss by 30%. Although the sample size was limited, it seems that the creation of a biologic width affects peri-implant bone loss to a significant extent and that platform switching is only effective when the mucosal thickness allows the establishment of a biologic width. Keywords: bone loss, implant, platform switching, southern implants, wide body