AbstractExcess body mass index (BMI) is associated with a higher risk of at least 13 cancers, but it is usually measured at a single time point. We tested whether the overweight‐years metric, which incorporates exposure time to BMI ≥25 kg/m², is associated with cancer risk and compared this with a single BMI measure. We used adulthood BMI readings in the Atherosclerosis Risk in Communities (ARIC) study to derive the overweight‐years metric. We calculated associations between the metric and BMI and the risk of cancers using Cox proportional hazards models. Models that either included the metric or BMI were compared using Harrell's C‐statistic. We included 13,463 participants, with 3,876 first primary cancers over a mean of 19 years (SD 7) of cancer follow‐up. Hazard ratios for obesity‐related cancers per standard deviation overweight‐years were 1.15 (95% CI: 1.05–1.25) in men and 1.14 (95% CI: 1.08–1.20) in women. The difference in the C‐statistic between models that incorporated BMI, or the overweight‐years metric was non‐significant in men and women. Overweight‐years was associated with the risk of obesity‐related cancers but did not outperform a single BMI measure in association performance characteristics.