OBJECTIVES: Evaluate the relationship between body mass index (BMI) and health claims costs over the last decade, assess the strength and nature of the relationship between BMI and costs, and identify comorbidities that may drive any increased costs. METHODS: Using 2001-2011 claims data for employees participating in annual health appraisals, annual paid claims costs were calculated. One-part negative binomial models were fit to evaluate the relationship between BMI and costs, controlling for age, gender, race/ethnicity, and calendar year period. RESULTS: The relationship between increasing BMI and increasing health claims costs is gradual and starts already at a BMI of 19. The nature of the relationship did not change notably over time. The most important obesity-related comorbidities, expressed as percent increase in cost per BMI unit, was cardiovascular disease (males 10.53, 95% CI [6.46, 14.77], females 4.27, 95% CI [1.25, 7.38), while cardiovascular agents (7.23, 95% CI [6.08, 8.39]) were the most important driver of pharmacy costs. CONCLUSION: In contrast to recent evidence relating to effects on mortality, we observed a gradual increase in health claims costs starting at the low end of the recommended BMI range.