Background. Intensive insulin treatment effectively delays the onset and slows the progression of microvascular complications in insulin-dependent diabetes mellitus (IDDM). Variable adherence to insulin treatment is thought to contribute to poor glycaemic control, diabetic ketoacidosis, and brittle diabetes in adolescents and young adults with IDDM. We assessed the association between the prescribed insulin dose and the amount dispensed from all community pharmacies with the Diabetes Audit and Research in Tayside Scotland (DARTS) database. Methods. We studied 89 patients, mean age 16 (SD 7) years, diabetes duration 8 (4) years, and glycosylated haemoglobin (HbA(1c)) 8.4 (1.9)%, who attended a teaching hospital paediatric or young-adult diabetes clinic in 1993 and 1994. The medically recommended insulin dose and cumulative volume of insulin prescriptions supplied were used to calculate the days of maximum possible insulin coverage per annum, expressed as the adherence index. Associations between glycaemic control (HbA(1c)), episodes of diabetic ketoacidosis, and all hospital admissions for acute complications and the adherence index were modelled. Findings. Insulin was prescribed at 48 (19) IU/day and mean insulin collected from pharmacies was 58 (25) IU/day. 25 (28%) of the 89 patients obtained less insulin than their prescribed dose (mean deficit 115 [68; range 9-246] insulin days/annum). There was a significant association between HbA(1c) and the adherence index (R2 = 0.39; p < 0.001). In the top quartile (HbA(1c) > 10%), 14 (64%) of individuals had an adherence index suggestive of a missed dose of insulin (mean deficit 55 insulin days/annum). There were 36 admissions for complications related to diabetes. The adherence index was inversely related to hospital admissions for diabetic ketoacidosis (p < 0.001) and all hospital admissions related to acute diabetes complications (p = 0.008). The deterioration in glycaemic control observed in patients aged 10-20 years was associated with a significant reduction (p = 0.01) in the adherence index. Interpretation. We found direct evidence of poor compliance with insulin therapy in young patients with IDDM. We suggest that poor adherence to insulin treatment is the major factor that contributes to long-term poor glycaemic control and diabetic ketoacidosis in this age group.