Background <p>Lifestyle interventions can reduce the risk of becoming an overweight adult and reduce the risk of health problems. The aim was to investigate the effects of obesity treatment on BMI-for-age, metabolic markers, and quality of life in children with overweight or obesity.</p> Methods <p>A randomized 1-year lifestyle intervention, including appointments every 3&#xa0;months, a total of 5 sessions. A multidisciplinary team consisting of a pediatrician, a nurse, and a clinical nutritionist conducted the study. The participants were 6–12-year-old children with overweight or obesity in the Hyvinkää area, Finland. The inclusion criteria were weight for height at least + 40% or ≥ 30% and ascending growth curve. Participants were randomized to the intervention group (<i>n</i> = 21) or the control group (<i>n</i> = 19). The intervention group received lifestyle counseling with an emphasis on nutritional guidance. The control group continued in the primary care and did not have any intervention during the study period. Laboratory values and anthropometric measurements were measured at baseline and at 12&#xa0;months. In addition, quality of life was evaluated by a questionnaire.</p> Results <p>In the intervention group fasting plasma glucose decreased statistically significantly (<i>p</i> = 0.017) and the difference in the change between the groups approached statistical significance (<i>p</i> = 0.080). No other significant changes were found between the intervention and the control groups at the end of the intervention. The increase of BMI-for-age stopped in both groups.</p> Conclusion <p>Encouragement to healthy lifestyle can improve glucose metabolism in children with obesity and to stop excessive weight gain.</p> Trial registration <p>18.12.2018 (§142/2018, HUS/2666/2018).</p> <p>ClinicalTrials.gov Identifier: NCT06126679.</p>