Although osteonecrosis (ON) is a common sequel after childhood acute lymphoblastic leukaemia treatment and may cause debilitating symptoms, its prognosis remains underexplored. We describe the radiological evolution of ON lesions in a Finnish patient cohort treated according to the The Nordic Society of Paediatric Haematology and Oncology (NOPHO) ALL2008 protocol. We aimed to identify the factors influencing the outcome of ON. We collected data from 37 patients diagnosed with ON treated in five tertiary centres. We analysed magnetic resonance imaging scans containing 235 ON lesions (109 affecting joints) and graded them using the Niinimäki classification system. The mean follow-up time from an ON diagnosis was 3.3 years (SD 3.4 range: 0.04–13.5). Among the lesions with follow-up scans, 55% remained stable, 35% resolved, 8% improved to lower grade and 2% progressed. Joint collapse was observed in 18 joint lesions (17%). Factors associated with unfavourable outcomes were female sex, older age at diagnosis and haematopoietic stem cell transplantation (HSCT). The chance for spontaneous resolution of ON was lower in females (adjusted odds ratio [aOR] 10.3, 95% CI 2.0–52.6) and decreased with age (aOR 1.4, 95% CI 1.1–1.7), whereas HSCT was associated with joint collapse already at ON diagnosis (aOR 8.3, 95% CI 2.6–27.0).