Morning hyperglycaemia remains a challenge to conventional insulin regimens. Eighteen adolescents participated in a one year crossover study to examine the effect of delaying the evening intermediate acting insulin from before the evening meal to bedtime. This three injection regimen caused slightly higher blood glucose concentrations in the early part of the night, and lower concentrations in the morning, but no overall change in glycosylated haemoglobin concentrations (HbA1c). Seasonal change accounted for substantially more of the variance in HbA1c concentrations than did the regimen change. The three injection regimen did not alter the frequency of hypoglycaemic episodes. Metabolic control on both regimens might have been improved by more intensive monitoring and medical attention. This study suggests that factors beyond medical control, such as seasonal variation, may contribute more to the control of diabetes in adolescents than changes in conventional insulin regimens, particularly when unaccompanied by intensive monitoring.