Abstract Background Asian population has different body mass index (BMI) profile compared to Caucasian population. However, the effect of obesity and body weight gain in Asian colorectal cancer patients treated with adjuvant chemotherapy has not been studied thus far. Methods We have analyzed the association between disease-free survival (DFS) and obesity/body weight change during treatment in Korean stage III or high-risk stage II colorectal cancer patients treated with adjuvant 5-fluorouracil/ leucovorin/oxaliplatin. BMI was classified according to WHO Asia-Pacific classification. Weight change was calculated by comparing body weights measured at the last chemotherapy cycle and before surgery. Results Among a total of 522 patients, 35.7 % of patients were obese (BMI ≥ 25 kg/m 2 ) and 29.1 % were overweight (BMI, 23–24.9 kg/m 2 ) before surgery. 18.0 % of patients gained ≥ 5 kg and 26.1 % gained 2–4.9 kg during the adjuvant chemotherapy period. Baseline BMI or body weight change was not associated with DFS in the overall study population. However, body weight gain (≥5 kg) was associated with inferior DFS (adjusted hazard ratio 2.04, 95 % confidence interval 1.02–4.08, p  = 0.043) in overweight and obese patients (BMI ≥ 23.0 kg/m 2 ). Conclusion In Korean colorectal cancer patients treated with adjuvant FOLFOX chemotherapy, body weight gain during the treatment period has a negative prognostic influence in overweight and obese patients.