Purpose: The purpose of this randomised controlled trial is to compare the efficacy of two techniques for enhancing peri-implant keratinised mucosa: acellular dermal matrix allograft versus free gingival grafts. Materials and methods: Thirty-six patients having implant sites with less than 1.5 mm of keratinised mucosa width were randomly assigned to two groups. Thirty-six implants placed in 18 patients received acellular dermal matrix allografts (ADM group), while 36 implants placed in 18 individuals received free gingival grafts (FGG group). Plaque index (PI), gingival index (GI), probing depth (PD) and the width of attached mucosa (WAM) were measured at baseline and at 1, 3 and 6 months following surgery. Results: WAM in the FGG group was significantly greater than the ADM group at 3 (P = 0.026) and 6 months (P 0.001). In the FGG group, final gain of WAM was greater (1.58 mm in ADM group, 2.57 mm in FGG group) (P 0.001) and postoperative relapse was smaller (2.68 mm in the ADM group, 1.73 mm in the FGG group) (P 0.001). PI and GI scores were greater in the ADM group at 6 months (P = 0.016 and P = 0.61, respectively). The FGG group demonstrated a greater PD value at 3 months (P 0.001), however there was no significant difference between the groups at 6 months (P = 0.317). Conclusions: Although ADM allografts are capable of increasing the width of peri-implant keratinised mucosa, FGGs seem to be more effective. ADM allografts may be the application of choice at implant sites in need of major grafts and in patients where a donor site should be avoided for medical or psychological reasons. Keywords: acellular dermal matrix, free gingival graft, peri-implant keratinised mucosa