Background: Studies have reported conflicting results on association between body mass index (BMI) and colorectal cancer-specific mortality and all-cause mortality in colorectal cancer patients. Methods: We performed a meta-analysis of 13 prospective cohort studies to identify the association between BMI (pre-diagnosis BMI or post-diagnosis BMI) and mortality outcome measures. Random-effects meta-analyses were performed to determine the risk ratio (RR) and 95% confidence interval (CIs). The analysis included 52,278 patients followed up over a period ranging from 4.9 to 20 years (median: 9.9 years). Results: We found that pre-diagnosis underweight was associated with increased all-cause mortality (RR: 1.62, 95% CI: 1.18-2.23, p<0.01) and obesity (BMI ≥ 30 kg/m²) was associated with increased colorectal cancer-specific mortality (RR: 1.20, 95% CI: 1.07-1.36, p<0.01) and all-cause mortality (RR: 1.24, 95% CI: 1.12-1.38, p<0.01). On the other hand, post-diagnosis underweight (RR: 1.33, 95% CI: 1.19-1.47, p<0.01) and class II/III obesity (BMI ≥ 35 kg/m²; RR: 1.13, 95%CI: 1.03-1.14, p<0.01) were associated with significantly increased all-cause mortality. When subgroup analysis according to gender was conducted, post-diagnosis obesity was associated with increased all-cause mortality (RR: 1.131, 95% CI: 1.054-1.213, p<0.01) in women only. Conclusions: Being obese prior to diagnosis of CRC was associated with CRC specific mortality and all-cause mortality, while being underweight and obesity after diagnosis was associated with increased all-cause mortality. Maintaining a normal body weight should be recommended to all individuals including colorectal cancer patients. This study was supported by National Research Foundation of Korea (2013 K2A1A2054437).