Background: The purpose of this study was to evaluate the expression of inducible nitric oxide synthase (iNOS) in the gingival tissues of periodontitis patients with and without type 2 diabetes to assess whether NO plays a role in the severity of periodontitis in patients with diabetes. Patients with diabetes and healthy patients were used as controls. Methods: A total of 80 patients were evaluated in four groups (with 20 subjects each): patients with chronic periodontitis and diabetes (12 males and eight females; mean age, 52.1 ± 6.9 years), patients with chronic periodontitis who were otherwise healthy (12 males and eight females; mean age, 43.1 ± 8.9 years), periodontally healthy patients with diabetes (12 males and eight females; mean age 50.9 ± 6.3 years), and systemically and periodontally healthy control subjects (12 males and eight females; mean age 29.8 ± 9.2 years). Periodontal parameters were recorded. Immunohistochemistry was used to detect inflammation and iNOS expression in gingival tissues. Results: Although periodontal parameters were slightly higher in periodontitis compared to diabetic periodontitis, immunohistochemical parameters were higher in diabetic periodontitis compared to periodontitis. All periodontal parameters were higher in patients with periodontitis and with/without diabetes compared to controls and patients with diabetes. All immunohistochemical parameters were higher in patients with diabetes and periodontitis compared to patients with only diabetes or periodontitis, but there was no difference between the latter two groups. There was a correlation between the expression of iNOS and inflammatory cells in controls, patients with diabetes, and patients with periodontitis but not in patients with diabetes and periodontitis. Conclusions: Inflammation and iNOS expression were more prominent in the gingiva of the patients with both diabetes and periodontitis. However, iNOS expression did not seem to have an additional detrimental effect on the course of periodontitis in patients with diabetes compared to those with periodontitis alone. J Periodontol 2010;81:109-120.