Glycaemic control of patients with diabetes has been shown to affect their periodontal disease severity (Tervonen & Oliver 1993). On the other hand, few intervention studies had examined the effect of periodontal therapy on glycaemic control. Objectives: The aims of the study are to compare the effects of different treatment modalities on periodontal parameters and glycaemic control in patients with diabetes mellitus and to examine the effect of glycaemic control on treatment response. Methods: 68 adult diabetic subjects were randomly assigned to one of three groups: test (simple non-surgical periodontal therapy and oral hygiene), oral hygiene, and control (no treatment) groups. For all subjects, full mouth periodontal charting and blood samples were collected at baseline, followed by 3 months. Results: There was a significant improvement in plaque and bleeding scores of those in the Oral hygiene and test group when compared with the control group (ANOVA P<0.05). The control group also showed some improvement although less than that found in the other 2 groups. No significant difference was noted in glycaemic control following intervention in all 3 groups. When the treatment responses were compared with the level of glycaemic control (HbA1c >or <8%), no appreciable differences were found. Conclusion: While simple periodontal therapy improves the periodontal health of subjects, it did not significantly improve glycaemic control on the short term. Glycaemic control did not seem to affect outcome of therapy. This study is supported by NMRC Grant 222-000-016-223