70 Background: The efficacy and tolerability of modern cytotoxic chemotherapy regimens used in malnourished metastatic colorectal cancer patients is uncertain. The aim of this study was to investigate the effect of malnutrition on efficacy and tolerability of cytotoxic chemotherapy and overall survival in mCRC patients. Methods: In this multicenter study, demographic, oncologic and nutritional data were collected prospectively from mCRC patients. Nutritional status were evaluated on the basis of NRI, BMI and WL before the first chemotherapy, after the first and second chemotherapy. To determine the inter-treatment weight loss toxicity assessment was included to theese parameters after each chemotherapy. NRIs were examined in 3 categories as ‘no malnutrition’ (NRI >97.5), ‘moderate malnutrition’ (97.5 ≥ NRI ≥83.5) or ‘severe malnutrition’ (NRI <83.5). Response to treatment and drug-induced toxicities were assessed based on RECIST 1.1 and CTCAE version 4.0 respectively. Results: 137 mCRC patients were prospectively included. Median age was 48 (range 18-83). Primary location was colon in 66% of patients, 84% of them source was left colon. Malnutrition was detected in 39% of the cases. Response rate to treatment was 24 %. Moderate / severe malnutrition was associated with multipl site of metastases, WHO PS of 1, over the median value of CEA/CA 19-9 levels (p=0.003, p=0.03, p<0.001, and p=0.02; respectively). Hypoalbuminemia and moderate/severe malnutrition were associated with all types of toxicity (p<0.001 and p<0.001). Moderate/severe malnutrition was associated with thrombocytopenia, and diarrhea following chemotherapy predominately, (p=0.02 and p=0.04; respectively). In moderate/severe malnutrition group median overall survival was prominently shorter than those with no malnutrition [6.6 moths (95 %CI, 5.6-7.6) vs 11.9 moths (95 % CI, 11.1-12.7) respectively, p<0.001]. Conclusions: Our study showed that moderate/severe malnutrition in mCRC patients was associated with decreased overall survival and increased chemotherapy toxicity.