We evaluated the effects of nonsurgical periodontal therapy in 100 patients with type 2 diabetes and chronic periodontitis. The participants were classified as having good (n = 48) or poor (n = 52) glycemic control and were further randomly allocated to receive either scaling and root planning treatment group or no treatment (n = 50 each). The effect of nonsurgical periodontal therapy was compared among diabetic patients with good glycemic control, those with poor glycemic control, and 25 nondiabetic individuals. Periodontal and metabolic status was recorded at baseline, 3 months, and 6 months. In patients receiving treatment, periodontal parameters significantly improved and HbA1c decreased by 10.8%. Improvements in gingival index and bleeding on probing were greater in the nondiabetic participants and the treated patients with good glycemic control than in the treated patients with poor glycemic control (P < 0.05). Regression analysis showed that improvement in periodontal status was independently associated with glycemic improvement. Nonsurgical periodontal therapy improved glycemic control and periodontal health in patients with type 2 diabetes. However, patients with poor baseline glycemic control had less clinical improvement than did those without diabetes and those with good glycemic control.