Obesity is associated with poor outcomes in childhood acute lymphoblastic leukaemia (ALL). We explored whether severe treatment-related toxicity and treatment delays could explain this observation. This study included 1 443 children aged 2·0–17·9 years with ALL treated with the Nordic Society of Pediatric Haematology and Oncology (NOPHO) ALL2008 non-high-risk protocol. Prospective treatment-related toxicities registered every three-month interval were used. Patients were classified according to sex- and age-adjusted international childhood cut-off values, corresponding to adult body mass index: underweight, <17 kg/m²; healthy weight, 17 to <25 kg/m²; overweight, 25 to <30 kg/m²; and obese, ≥30 kg/m². Obese children had a higher incidence rate ratio (IRR) for severe toxic events {IRR: 1·55 [95% confidence interval (CI) 1·07–2·50]}, liver and kidney failures, bleeding, abdominal complication, suspected unexpected severe adverse reactions and hyperlipidaemia compared with healthy-weight children. Obese children aged ≥10 years had increased IRRs for asparaginase-related toxicities compared with healthy-weight older children: thromboses [IRR 2·87 (95% CI 1·00–8·21)] and anaphylactic reactions [IRR 7·95 (95% CI 2·15–29·37)] as well as higher risk for truncation of asparaginase [IRR 3·54 (95% CI 1·67–7·50)]. The high prevalence of toxicity and a higher risk of truncation of asparaginase may play a role in the poor prognosis of obese children aged ≥10 years with ALL.