Indiana University-Purdue University Indianapolis (IUPUI) / Purpose: Obesity is a growing public health problem and the second most preventable cause of death in the US. Obesity has been linked as a risk factor for several cancers. However, there are limited studies that have examined the roles of metabolic syndrome (MetS) and C-reactive protein (CRP), as well as change in body composition from early adulthood to late adulthood on the risk of cancer. The overall objective of this dissertation was to determine the association of obesity and obesity-related markers with breast and colorectal cancer occurrence and mortality. Methods: Three datasets were used. The first study used 4,500 asymptomatic adults who were surveyed during a colorectal cancer screening study. The second study was based on the National Health and Nutrition Examination Survey (NHANES) 2005-2010. The dataset had 172 breast cancer survivors and 2,000 women without breast cancer. The last manuscript resulted from the NHANES follow-up study (NHANES III). A total of 120 cancer deaths from breast and colorectal deaths were identified from 10,103 women aged 18 years or older. Results: Overall, obesity and obesity related markers were associated with breast and colorectal cancer occurrence and mortality. BMI change and WC change were positively associated with increased risk of advanced colorectal neoplasia (AN). WC measures (both static and dynamic) were generally a better predictor of AN compared to BMI. In the second study involving breast cancer survivors, neither MetS nor CRP were associated with having a breast cancer diagnosis. Also, none of the individual components of MetS (WC, Triglycerides, HDL, fasting blood glucose and blood pressure) were associated with a breast cancer diagnosis. In the last study, MetS was associated with increased risk of mortality from obesity-related cancers. In addition, all components of MetS, except dyslipidemia, were associated with increased risk of mortality for the obesity-related cancers. Conclusion: Obesity expressed in terms of BMI and WC, or their change, MetS and CRP are important factors in regard to the occurrence, survivorship and mortality of breast and colorectal cancer. The results of this research underscore the importance of maintaining a healthy weight.