Background: Childhood cancer survivors are at risk of developing skeletal comorbidities later in life. We aimed to assess risk factors for low and very low bone mineral density (BMD), and the risk of and risk factors for any fractures and vertebral fractures in a national cohort of Dutch adult childhood cancer survivors. Methods: In this cross-sectional study, we used data from the DCCSS LATER cohort, which comprised individuals who were alive for at least 5 years after diagnosis of childhood cancer (ie, histologically confirmed malignancies or Langerhans cell histiocytosis), were diagnosed before the age of 19 years, and who had been treated at one of seven Dutch paediatric oncology centres between 1963 and 2002 (hereafter referred to as survivors). For this study, we invited survivors aged 18–45 years, who were alive as of Oct 10, 2016, living in the Netherlands, and who were deemed eligible by their treating physician to participate. We assessed BMD using dual-energy x-ray absorptiometry (DXA). Self-reported fractures that occurred at least 5 years after cancer diagnosis were assessed using available medical history and compared with population-level data from the Swedish national registry. We assessed vertebral fractures in a subset of participants using a vertebral fracture assessment. We assessed associations between the occurrence of low (Z-score of ≤–1) or very low (Z-score of ≤–2) BMD, fractures, and vertebral fractures and demographic, treatment-related, endocrine, and lifestyle-related factors using logistic regression analysis. Findings: Between April 29, 2016, and Jan 22, 2020, 3996 (64·8%) of 6165 individuals from the DCCSS LATER cohort were invited to participate, of whom 2003 (50·1%) were enrolled (mean age at participation was 33·1 years [SD 7·2], 966 [48·2%] were female, and 1037 [51·8%] were male [data on ethnicity and race were not available due to national policies]). 1548 (77·3%) had evaluable DXA scans for assessment of BMD, 1892 (94·5%) provided medical history of fractures, and 249 (12·4%) were assessed for vertebral fractures. 559 (36·1%) of 1548 had low BMD at any site, and 149 (9·6%) had very low BMD at any site. The standardised incidence ratio of any first fracture was 3·53 (95% CI 3·06–4·06) for male participants and 5·35 (4·46–6·52) for female participants. 33 (13·3%) of 249 participants had vertebral fractures. Male sex, underweight, high carboplatin dose, any dose of cranial radiotherapy, hypogonadism, hyperthyroidism, low physical activity, and severe vitamin D deficiency were associated with low BMD at any site and male sex, underweight, cranial radiotherapy, growth hormone deficiency, and severe vitamin D deficiency were associated with very low BMD at any site. Additionally, male sex, former and current smoking, and very low lumbar spine BMD were associated with any fractures, whereas older age at follow-up, previous treatment with platinum compounds, growth hormone deficiency, and low physical activity were specifically associated with vertebral fractures. Interpretation: Survivors of childhood cancer are at increased risk of any first fracture. Very low lumbar spine BMD was associated with fractures, highlighting the importance of active BMD surveillance in high-risk survivors (ie, those treated with cranial, craniospinal, or total body irradiation). Moreover, our results indicate that intensive surveillance and timely interventions for endocrine disorders and vitamin deficiencies might improve bone health in childhood cancer survivors, but this needs to be assessed in future studies. Funding: Children Cancer-free Foundation (KiKa), KiKaRoW, and ODAS foundation.