The aim of the present study was to monitor clinical, microbiological, medical, and immunological effects of non-surgical periodontal therapy in diabetics and healthy controls. 20 IDDM (insulin dependent, n = 7) or NIDDM (non-insulin dependent, n = 13) diabetic patients (median duration 11.5 years, range of HbA1C: 4.4-10.6%) with moderate to advanced periodontal disease and 20 matched healthy control patients, were subjected to supragingival pretreatment and subsequent subgingival therapy. Periodontal examinations (API, PBI, BOP, PPD, PAL), microbiological examinations (culture), medical routine examinations, and immunological examinations (oxidative burst response of PMNs to TNF-alpha and FMLP) were performed at baseline, 2 weeks after supragingival, and 4 months after subgingival therapy. 4 months after completion of non-surgical therapy, the following compared to baseline significant (p < or = 0.05) changes (delta) of clinical parameters (median) were found in diabetic patients versus control patients: deltaAPI (30.4% versus 36.3%), deltaPBI (22.9% versus 24.2%), deltaBOP (39.5% versus 46.9%). The median % per patient of pockets with PPD > or = 4 mm decreased from 41.9% to 28.3% in diabetics, and from 41.6% to 31.8% in controls. Microbiologically, similar reductions of periopathogenic bacteria were found in diabetics and controls. Neither periodontal data nor the oxidative burst response of PMNs showed any significant difference (p > 0.05) between diabetics and control patients. In this study, periodontal therapy had no significant influence on medical data of diabetics. In conclusion, this study indicates that metabolically well-controlled diabetics might respond to non-surgical periodontal therapy as well as healthy control patients.