The aim of this study was to evaluate the outcomes of lateral guided bone regeneration (GBR) using a novel resorbable synthetic polyethylene-glycol/methacrylate (PEG/MET) membrane compared to a non-cross-linked collagen membrane (CM). Twenty-eight patients with a potential implant site exhibiting insufficient bone width of $\leq$ 5 mm were included. Ridge width was measured intraoperatively at 1 mm and 4 mm apical to the crest and via cone-beam computed tomography at baseline and 6 months following GBR using either a PEG/MET or a CM in conjunction with an allograft. During implant placement, core biopsies were harvested and analyzed histomorphometrically. Width changes were calculated. Differences between groups were analyzed using two-sided t-test and Mann-Whitney U-test. The PEG/MET membrane was moldable and exhibited higher strength and stability compared to the CM. Nevertheless, it displayed higher exposure rate of 12/15, compared to 2/13 in the CM sites. At the time of implant insertion, 6 months following GBR, significant gain in bone width was observed in both groups. Mean ridge width at 1 mm and 4 mm apical to the crest was increased significantly from 2.06 $\pm$ 0.77 mm and 3.84 $\pm$ 1.23 mm to 3.84 $\pm$ 1.52 mm and 6.06 $\pm$ 2.03 mm ($p=$ 0.0006 and $p=$ 0.0009, respectively), with no clinical or radiographic differences between groups. Experimental sites contained more residual scaffold material than the controls (17.4 $\pm$ 3.3% and 8.6 $\pm$ 2.0%, $p=$ 0.0566). However, bone and connective tissue area fraction were not statistically different between the groups. Overall, despite the higher exposure rate, the new PEG/MET membrane was as successful as a standard collagen membrane in lateral GBR and may have potential use in bone augmentation procedures. This study suggests the feasibility of synthetic membranes, which are not associated with disease transmission, as an attractive alternative to the commonly used CM of bovine or porcine origin.