Inflammatory processes in the tissues around the implant are defined as peri-implant diseases, which can be divided into peri-implant mucositis and peri-implantitis. The clinical features of peri-implant mucositis correspond to those of gingivitis, including symptoms of bleeding and swelling, while for the diagnosis of peri-implantitis, in addition to the listed symptoms, bone resorption must be confirmed by X-ray. There are systemic and local factors for the development of peri-implant disease. When treating patients with diabetes mellitus, it is important to assess the patient's preoperative risk factors, control of hyperglycemia, level of oral hygiene, and width and thickness of the keratinized peri-implant mucosa, which can prevent the conditions that may jeopardize the dental implant. The patient, 57 years old woman, non - smoker, suffering from type 2 diabetes mellitus under internal control, came for regular implant examination after 6 months, and changes in the attached gingiva around the implant at the position of teeth 43 and 33 were noticed. Clinical examination revealed inflammatory signs of redness, swelling, and bleeding on probing. The observed symptoms are characteristic for the diagnosis of peri-implant mucositis or peri-implantitis in the case of implants at position 33 with X-ray finding of bone loss. The lack of a sufficient volume of keratinized peri-implant mucosa leads to easier accumulation of bacteria and inflammatory changes due to the difficulty of proper oral hygiene. Peri-implant disease therapy was approached according to the CIST protocol to prepare the patient for the free gingival graft surgery to increase the width of the keratinized peri-implant mucosa. After healing, an increase in the volume of keratinized gingiva around the implant is expected, thus preventing the occurrence of inflammatory processes with proper oral hygiene.