Background and objectives:Acute lymphoblastic leukaemia (ALL) is associated with a cytokine imbalance and oxidative stress, which can be aggravated by malnutrition. Malnutrition, defined by the World Health Organisation (WHO) as obesity or undernutrition, can affect treatment complications and outcomes. Therefore, we aimed to analyse the change in the body mass index (BMI)z-score during induction, as well as evaluate the impact of childhood malnutrition on fevers at an ALL presentation and early response to therapy.Methods: An observational cohort study of 50 consecutive children with ALL, diagnosed in 2019–2022, was performed. Patients were divided into age groups of 0–5, 6–11, and 12–17 years. BMI-for-agez-scores were used to define undernutrition and overnutrition according to WHO growth standards.Results:The number of patients with an abnormal BMI increased from 3 (6%) at diagnosis to 10 (20%) at the end of induction (from 2 (4%) to 6 (12%) in overweight/obese, and from 1 (2%) to 4 (8%) in underweight patients). At the end of induction, all overweight/obese patients were 0–5 years old. On the other hand, a statistically significant decrease in the mean BMIz-score among patients aged 12–17 was observed (p= 0.005). The mean BMIz-score differed statistically significantly among children aged 0–5 presenting with and without fever (p= 0.001). The minimal residual disease (MRD) level at the end of induction was not related to BMI at diagnosis.Conclusions: Despite the use of steroids, adolescents are prone to losing weight during an ALL induction, in contrast to preschool children, who tend to gain weight under the same treatment. BMI at diagnosis was related to a fever of ≥38 °C (at ALL presentation) in the 0–5 age group. The results emphasise the importance of careful nutritional status monitoring, with younger and older children as important target groups for weight gain and weight loss interventions, respectively.