Variation in the periodontal health status and the response to oral hygiene education, scaling and root planing were studied in 36 subjects with type-1 diabetes mellitus (DM) and in 10 non-diabetic control subjects. The age range of the subjects was 24-36 years. The diabetic group was divided into 3 subgroups based on the levels of glycosylated hemoglobin (HbAlc) over a 3 year period and the presence of diabetic complications as follows: (D1) subjects with good metabolic control and no complications (n=13), (D2) subjects with varying metabolic control with/without retinopathy (n=15) and (D3) subjects with severe diabetes, i.e., with poor long-term control and/or multiple complications (n= 8). Clinical measurements (plaque, subgingival calculus, probing pocket depth, bleeding after probing and clinical attachment level) were performed at the baseline and 4 weeks and 6 and 12 months after periodontal therapy. The between-group comparisons were made using the Student t-test and ANOVA. Based on the plaque scores, the oral hygiene status was similar in all groups during the whole study. No statistically-significant differences in the periodontal health status could be found between the diabetic group as a whole and the non-diabetic controls at any examination. The level of periodontal health of the diabetics with good control and no complications (D1) and those with moderate control with/without retinopathy (D2) was on the same level with that seen in the non-diabetic controls. Our findings of the significantly higher extent of al > or =2 mm at the baseline and the fast recurrence of pd > or =4 mm during the longitudinal study in diabetic subjects with poor metabolic control and/or multiple complications (D3) indicate increased periodontal breakdown as a complication of DM in these subjects. To be able to assess the periodontal prognosis and the need for periodontal therapy on an individual basis,the clinical practitioner should be well aware of the diabetic status of his/her patients.