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Background Obesity, assessed by body mass index (BMI), is an established risk factor for 13 cancers. We aimed toidentify further potential obesity-related cancers and to quantify their association with BMI relative to that ofestablished obesity-related cancers.

Methods Using Cox regression models on 4,142,349 individuals in Sweden (mean age 27.1 years at weight mea-surement), we calculated hazard ratios (HRs) for the association between BMI and the risk of 122 cancers and cancersubtypes, grouped by topography and morphology. Cancers with a positive association (i.e., HR >1) at an alpha-level of0.05 for obesity (BMI >= 30 kg/m(2)) vs. normal weight (BMI 18.5-24.9 kg/m(2)) or per 5 kg/m2higher BMI, for whichobesity is not an established risk factor, were considered potentially obesity related.

Findings After 100.2 million person-years of follow-up, 332,501 incident cancer cases were recorded. We identified 15cancers in men and 16 in women as potentially obesity related. These were cancers of the head and neck,gastrointestinal tract, malignant melanoma, genital organs, endocrine organs, connective tissue, andhaematological malignancies. Among these, there was evidence of differential associations with BMI betweensubtypes of gastric cancer, small intestine cancer, cervical cancer, and lymphoid neoplasms (P values forheterogeneity in HRs <0.05). The HR (95% confidence interval) per 5 kg/m(2)higher BMI was 1.17 (1.15-1.20) in men and 1.13 (1.11-1.15) in women for potential obesity-related cancers (51,690 cases), and 1.24 (1.22-1.26) in menand 1.12 (1.11-1.13) in women for established obesity-related cancers (84,384 cases).

Interpretation This study suggests a large number of potential obesity-related cancers could be added to alreadyestablished ones. Importantly, the magnitudes of the associations were largely comparable to those of the alreadyestablished obesity-related cancers. We also provide evidence of specific cancer subtypes driving some associationswith BMI. Studies accounting for cancer-specific confounders are needed to confirm thesefindings.