Despite technological advances that have significantly improved dental implant osseointegration and survival rates, biological complications such as peri-implantitis and peri-implant mucositis are common around the implant. The active form of vitamin D, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), can be induced by the expression of leucine-lysine-37 (LL-37), a human antimicrobial peptide in the cathelicidin family. Studies have shown that 1,25(OH)2D3 and its precursor, 25(OH)D3, significantly reduce inflammation by suppressing the release of receptor activator nuclear factor kappa b ligand (RANKL), tumor necrosis factor-alpha (TNF-α), interleukin (IL)-1, and IL-6, and by preventing alveolar bone loss. To date, many cytokines have been investigated in peri-implant sulcus fluid (PIOS). The lack of a direct study investigating vitamin D and LL-37 levels in PIOS constitutes the unique value of this project. This study aims to compare LL-37 and 25(OH)D3 levels in PIOS and examine their correlation with markers such as IL-6, IL-10, and MMP-8, as well as clinical parameters. Volunteers aged 18-65 who have received dental implants at the Recep Tayyip Erdoğan University Faculty of Dentistry and have been using their implant-supported prosthesis for at least one year and who have been informed about this study will be included in the study. The groups will be formed as peri-implant health, peri-implant mucositis, and peri-implantitis. The implants of the patients included in the study will be evaluated for probed pocket depth (PPD), clinical attachment level (CAL), suppuration (S), modified gingival index (mGI), plaque index (mPI), bleeding index (mBI), keratinized gingival amount, and attached gingival amount. PIOS and serum samples will be obtained from patients, and their relationship to periodontal status and each other will be statistically evaluated. This project may help discover new biomarkers that can be used in the early diagnosis of peri-implantar diseases. This study could contribute to earlier treatment, lower patient costs, and even the development of new treatment plans for these diseases.