Intensification of insulin therapy in the Diabetes Control and Complications Trial led to an improvement in the quality of diabetes care, which was accompanied, however, by a threefold increase in the risk of severe hypoglycaemia. The present trial, a long-term evaluation of a structured 5-day treatment and teaching programme (DTTP) for intensified insulin therapy, was performed to clarify factors determining HbA1c, the incidence of severe hypoglycaemia, diabetes knowledge and quality of life. Ninety-four Type 1 diabetic patients were examined at baseline and 4 years after participation in a DTTP. Comparison of baseline data with measurements at the 4-year follow-up examination showed that relative HbA1c (= HbA1c/mean normal) improved (1.9 +/- 0.51 vs 1.55 +/- 0.3*, p < 0.001, *excluding 4 patients with diabetes manifestation at baseline) and that frequencies of daily insulin injections (3.73 +/- 1.23 vs 4.9 +/- 0.69*, p < 0.001) and weekly blood glucose self-tests (6.6 +/- 10.1 vs 25.5 +/- 8.7*, p < 0.001) increased, whereas the incidence of severe hypoglycaemia (intravenous glucose, glucagon injection) remained stable (0.19 vs 0.24, p = 0.48). Patients with less diabetes knowledge had higher HbA1c levels and a higher incidence of severe hypoglycaemia. In the group of patients with severe hypoglycaemia, certain crucial gaps in diabetes knowledge were identified concerning the effects of physical activity, nutrition and long-term complications of diabetes. In multivariate analysis. The most important factor associated with HbA1c was diabetes knowledge which, however, was not influenced by educational level or other factors. Interventions, such as the identification of psychosocial factors which may interact with diabetes knowledge, quality of life and successful self-management of diabetes by patients, are needed to improve the efficacy of DTTPs and to prevent severe side effects such as hypoglycaemia.