Aims To assess the outcome of a Diabetes Treatment and Teaching Programme (DTTP) on glycated haemoglobin (HbA(1c)), severe hypoglycaemia (SH) and severe ketoacidosis (SKA) in adolescents and young adults with Type 1 diabetes. Methods Quality-assurance project with assessment of participants 1 year after participation in a DTTP (5-day inpatient course, groups <= 10 patients, fixed curriculum of education/training, introduction of dietary freedom). Before-after analyses of participants aged 12-15, 15-18, 18-21 and 21-24 years. Main outcome measures were HbA(1c), SH and SKA. Results For the 1592 participants, aged 12 to 24 years, mean age at enrolment was 19 +/- 3 years, mean duration of diabetes was 7.3 +/- 5.4 (range 0.3-24) years, mean baseline HbA(1c) declined from 8.8 +/- 2.3% to 8.1 +/- 2.0%. The incidence of SH was 0.31 vs. 0.11 events/patient/year; the incidence of SKA 0.17 vs. 0.07 events/patient/year. In mixed effects models taking into account effects of centres, age and diabetes duration, the mean difference was -0.64% [P < 0.001, 95% confidence interval (CI) -0.79 to -0.5] for HbA(1c), -0.2 events/patient/year (P < 0.0001, 95% CI -0.28 to -0.12) for SH and -0.1 events/patient/year (P < 0.0001, 95% CI -0.14 to -0.06) for SKA. Conclusions Adolescents and young adults with Type 1 diabetes benefit from participation in a standard DTTP for flexible, intensive insulin therapy and dietary freedom.