Gingival health (bleeding on probing) and oral hygiene (plaque percent) were assessed in 2 groups of children and adolescents with insulin-dependent diabetes mellitus (IDDM). 1st study group included 12 newly diagnosed diabetic children and adolescents (age range 6.3-14.0 years, 5 boys and 7 girls). They were examined on the 3rd day after initial hospital admission and at 2 weeks and 6 weeks after initiation of insulin treatment. Gingival bleeding decreased after 2 weeks of insulin treatment (37.8% versus 19.0%, p < 0.001, paired t-test), and remained at the same level when examined 1 month later while glucose balance was excellent. Another group (n = 80) of insulin-dependent diabetic children and adolescents (age range 11.7-18.4 years, 44 boys and 36 girls) with a mean duration of diabetes 6.0 years (range 0.3-15.0 years) were examined 2x at 3-month intervals. Subjects with poor blood glucose control (glycosylated haemoglobin, HbA1, values over 13%) had more gingival bleeding (46.3% on examination 1, 41.7% on examination 2) than subjects with HbA1 values less than 10% (mean gingival bleeding 35.2% and 26.9%, respectively) or subjects with HbA1 values between 10 to 13% (mean gingival bleeding 35.6% and 33.4%, respectively). Differences were significant on both examinations (p < 0.05, Anova), and remained significant after controlling the groups for differences in age, age at the onset of diabetes, duration of diabetes and pubertal stage (Ancova). Results were not related to differences or changes in dental plaque status, supporting the concept that imbalance of glucose metabolism associated with diabetes predisposes to gingival inflammation. An increase in gingival bleeding in association with hyperglycaemia suggests that hyperglycaemia-associated biological alterations, which lower host resistance toward plaque, have apparently taken place. Consequently, although not all gingivitis proceeds into a destructive periodontal disease, prevention of plaque-induced gingival inflammation should be emphasised, particularly in children and adolescents with poorly controlled diabetes.