Abstract Uncontrolled studies have suggested a beneficial effect of periodontal treatment on metabolic control of insulin-dependent diabetes mellitus (IDDM). We therefore conducted controlled single-blind studies, using current metabolic status indicators in IDDM subjects free of significant complications other than periodontal diseases. In the 1st study, 41 IDDM subjects with gingivitis and early periodontitis were randomly assigned to treatment (oral hygiene and scaling) or control groups. The study was completed by 16 experimental and 15 control subjects. Reassessment after 2 months showed a Hawthorne effect in the control group, and no difference between groups. However, further analysis showed a relationship between individual metabolic control variation and gingival inflammation. A 2nd study enrolled 23 IDDM subjects with advanced periodontitis, who were randomised to treatment (full initial therapy including root planing) or control groups. Only 1 subject failed to complete the study, owing to illness. In this study, a significant response to periodontal treatment was not accompanied by any improvement in metabolic control. These results support the concept that the effect of metabolic control may be predominant in the relationship between IDDM and periodontal health.