Objectives: The purpose of this study was to evaluate the effect of subgingival administration of doxycycline as an adjunct to periodontal therapy in type 1 diabetes mellitus (DM) patients. Material and methods: Twenty-two paired periodontal defects greater than or equal to5.0 mm were treated in 11 patients (35-55 years old). After initial therapy the sites were randomly assigned into test (scaling and root planing+subgingival administration of 10% doxycycline hyclate gel) or control (scaling and root planing+subgingival placebo gel) groups. The clinical parameters of clinical attachment level (CAL), probing depth (PD) and gingival margin level (GML) for recession determination were assessed at baseline, after 6 weeks, and 6, 9 and 12 months, using a computerized probe. Data were statistically evaluated using Duncan and F tests. Results: Between study group comparisons indicated PD reduction and CAL gain were greater in the test group than in the control group at 6 weeks and 6, 9 and 12 months but only statistically significant at 12 months (p<0.05). Within study group comparisons indicated statistically significant differences were found for CAL and PD values favouring the adjunctive doxycycline group from baseline to 6 weeks and 6, 9 and 12 months (p<0.05). Conclusions: These findings suggest that subgingivally delivered doxycycline hyclate produces additional favorable clinical results to periodontal therapy in type 1 DM patients.318648653