Aims: The objective of this randomised controlled trial was to compare the efficacy of two techniques for increasing the amount of keratinised mucosa around implants: free gingival grafts versus classic vestibuloplasty. Materials and methods: Sixty-four patients with 64 implants presenting keratinised mucosa 1.5 mm and showing signs of peri-implant mucositis were randomly assigned to two groups. Thirty-two implants received free gingival grafts (FGG group) while 32 implants were treated via vestibuloplasty (VP group). Plaque Index (PI), Gingival Index (GI), probing depth (PD) and the width of attached mucosa (WAM) were measured at baseline and 1, 3, 6 and 12 months following surgery. Results: WAM in the FGG group was significantly greater than the VP group at 3, 6 and 12 months (P = 0.000). In the FGG group, the final gain in WAM was greater (2.36 mm in FGG group, 1.15 mm in VP group) (P = 0.000) and the postoperative relapse was smaller (2.00 mm in FGG group, 3.06 mm in VP group) (P = 0.000). The VP group had higher PD values at 3, 6 and 12 months (P = 0.02, P = 0.024, P = 0.000, respectively). Conclusions: The application of FGG is a more predictable method for enhancing the width of attached mucosa in the vicinity of implants compared with classic vestibuloplasty. Palabras clave: attached mucosa, free gingival graft, keratinised mucosa, peri-implant health