This study aimed to increase the monounsaturated fat content in the diet of outpatient adolescents with type 1 diabetes and to examine the metabolic effects after 12 weeks. Twenty-three adolescents were randomly allocated to either a high monounsaturated fat diet or a control diet. Their mean age was 16.9 (S.D. 2.1) years and median HbA_1c was 9.1% [IQR 7.9–10.4%]. Dietary targets were not reached judged by their 4-day food diaries. However, the whole study group had a significant increase in monounsaturated fat as indexed by red cell phospholipid fatty acids (RCFAs), with an increase of n-9 RCFAs from 14.9% [IQR: 14.5–21.7%] to 21.7% [IQR: 18.8–25.6%] (P=0.002). Changes in n-9 RCFAs were inversely related to changes in HbA_1c (R²=0.26, P=0.02), such that a 10% increase in n-9 RCFAs corresponded to a 0.64% improvement (decrease) in HbA_1c. Changes in n-9 RCFAs were also inversely related to changes in plasma total cholesterol (R²=0.38, P=0.002) and plasma LDL cholesterol (R²=0.21, P=0.03). These changes were not associated with changes in insulin dose, body weight or physical activity. Overall, the results demonstrate that a modest increase in the monounsaturated fat content of an adolescent diet has the potential to improve glycaemic control and lipid profile.