The effects of the Dietary Approaches to Stop Hypertension (DASH) eating plan on childhood metabolic syndrome (MetS) and insulin resistance remain to be determined. The present study aimed to assess the effects of recommendations to follow the DASH diet v. usual dietary advice (UDA) on the MetS and its features in adolescents. In this randomised cross-over clinical trial, sixty post-pubescent adolescent girls with the MetS were randomly assigned to receive either the recommendations to follow the DASH diet or UDA for 6 weeks. After a 4-week washout period, the participants were crossed over to the alternate arm. The DASH group was recommended to consume a diet rich in fruits, vegetables and low-fat dairy products and low in saturated fats, total fats and cholesterol. UDA consisted of general oral advice and written information about healthy food choices based on healthy MyPlate. Compliance was assessed through the quantification of plasma vitamin C levels. In both the groups, fasting venous blood samples were obtained at baseline and at the end of each phase of the intervention. The mean age and weight of the participants were 14·2 (sd 1·7) years and 69 (sd 14·5) kg, respectively. Their mean BMI and waist circumference were 27·3 kg/m2 and 85·6 cm, respectively. Serum vitamin C levels tended to be higher in the DASH phase than in the UDA phase (860 (se 104) v. 663 (se 76) ng/l, respectively, P= 0·06). Changes in weight, waist circumference and BMI were not significantly different between the two intervention phases. Although changes in systolic blood pressure were not statistically significant between the two groups (P= 0·13), recommendations to follow the DASH diet prevented the increase in diastolic blood pressure compared with UDA (P= 0·01). We found a significant within-group decrease in serum insulin levels (101·4 (se 6·2) v. 90·0 (se 5·5) pmol/l, respectively, P= 0·04) and a non-significant reduction in the homeostasis model assessment for insulin resistance score (P= 0·12) in the DASH group. Compared with the UDA group, the DASH group experienced a significant reduction in the prevalence of the MetS and high blood pressure. Recommendations to follow the DASH eating pattern for 6 weeks among adolescent girls with the MetS led to reduced prevalence of high blood pressure and the MetS and improved diet quality compared with UDA. This type of healthy diet can be considered as a treatment modality for the MetS and its components in children.