Background <p>Adolescent Idiopathic Scoliosis (AIS) affects over 1% of adolescents and is often diagnosed during a sensitive developmental period, with repeated radiographs, potentially increasing cancer risk. Previous studies have suggested an association between scoliosis and breast cancer, but have been limited by small cohorts, lack of matched controls, and incomplete registry data. The aim of this study, therefore, was to assess the hazard ratio (HR) of cancer among women with scoliosis compared to age-matched controls using nationwide registry data.</p> Methods <p>This observational cohort study included all Danish women born 1967–1977 with a scoliosis diagnosis (ICD-8 735.0/735.2 or ICD-10 DM411), and four age-matched controls per case, identified through national health registers. Cancer outcomes were obtained from the Danish National Cancer Register, and Cox regression was used to estimate HRs, with time at risk beginning at age 12.</p> Results <p>Among 6,217 women (1,238 (20% with scoliosis), the mean follow-up time was approximately 38&#xa0;years. The HR (95% CI) for any cancer was 1.06 [0.85–1.31]. For breast cancer, HR was 1.25 [0.86–1.80], while HR for carcinoma in situ (CIS) of the breast was 13.62 [3.75–49.50]. Among 12 cases of ovarian cancer, the HR was 2.91 [0.92–9.18]. This study could not, however, discriminate between idiopathic and non-idiopathic scoliosis.</p> Conclusion <p>Women with scoliosis showed a significantly increased HR for CIS of the breast, and a non-significant trend toward increased risk of invasive breast and gynecologic cancers. Findings support further investigation into genetic or developmental links between scoliosis and cancer risk.</p> Key Points <p><UnorderedList Mark="Bullet"> <ItemContent> <p>Women with scoliosis are at a higher risk of getting cancer, namely of the breast.</p> </ItemContent> <ItemContent> <p>Pregnancy did not appear to have a protective effect.</p> </ItemContent> </UnorderedList></p>