Background. Despite the causal relationship between obesity and colon cancer being firmly established the effect of obesity on the course of cancer calls for further elucidation. The objective was to assess differences in clinicalpathological and psychosocial variables between obese and non-obese individuals with colon cancer. Patients and Methods. Prospective, multicentric, observational study conducted from 2015-2018. The sample comprised patients with stage II-III, resected colon cancer about to initiate adjuvant chemotherapy with fluoropyrimidine in monotherapy or associated with oxaliplatin and grouped into non-obese (BMI <30kg/m 2) or obese (≥30kg/m 2). Subjects completed questionnaires appraising quality of life (EORTC-QLQ-C30), coping (M-MAC), psychological distress (BSI-18), perceived social support (MSPSS), personality (BFI-10), and pain (BPI). Toxicity, chemotherapy compliance, 12-month recurrence and mortality rate data were recorded. Results. Seventy-nine of the 402 individuals recruited (19.7%) were obese. Obese subjects exhibited more comorbidities (≥2 comorbidities, 46.8% vs 30.3%, p=0.001) and expressed feeling slightly more postoperative pain (small size-effect). There was more depression, greater helplessness, less perceived social support from friends, and greater extraversion among the obese versus non-obese subjects (all p<0.04). The non-obese group treated with fluoropyrimidine and oxaliplatin suffered more grade 3-4 hematological toxicity (p=0.035), whereas the obese had higher rates of treatment withdrawal (17.7% vs 7.7%, p=0.033) and more recurrences (10.1% vs 3.7%, p=0.025). No differences in sociodemographic, quality of life, or 12-month survival variables were detected. Conclusion. Obesity appears to affect how people confront cancer, as well as their tolerance to oncological treatment and relapse. The Oncologist 2021;9999:• • Implications for Practice: Obesity is a causal factor and affects prognosis in colorectal cancer. Obese patients displayed more comorbidities, more pain following cancer surgery, worse coping, more depression, and perceived less social support than the non-obese. Severe hematological toxicity was more frequent among non-obese