Background: The present clinical trial study was designed to assess the effect of topical application of melatonin on serum levels of tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6) and C-reactive protein (CRP) in patients with diabetes and periodontal disease in comparison with healthy controls. Material and Methods: Serum levels of TNF-α and IL-6 were measured by enzyme-linked immunosorbent assay and CRP by nephelometry by using the proper commercial kits in 30 patients with diabetes and periodontal disease, and also in a control group of 30 healthy subjects. Periodontograms were performed using the Florida Probe®. Patients with diabetes were treated with a topical application of melatonin (1% orabase cream formula) once daily for 20 days. Healthy subjects were treated with a placebo orabase cream. Results: Patients with diabetes and periodontal disease had significantly higher mean levels of serum TNF-α, IL-6 and CRP than healthy subjects ( P < 0.001). Following topical melatonin application, there was a statistically signi - ficant decrease in the gingival index and pocket depth ( P < 0.001) as well as a significant decrease in IL-6 and CRP serum levels ( P < 0.001). Local melatonin application in patients with diabetes and periodontal disease resulted in a significant decrease in CRP and IL-6 serum levels as well as an improvement in the gingival index and pocket depth. Patients with periodontal disease had significantly higher serum CRP, IL-6 and TNF-α values by comparison with healthy subjects. Conclusions: We conclude that melatonin can modulate the inflammatory action of these molecules in periodontal patients