The present invention provides a method for predicting the therapeutic effect of chemotherapy using an antitumor agent that contains a,a,a-trifluorothymidine and 5-chloro-6-(1-(2-iminopyrrolidinyl)methyl) uracil hydrochloride in a molar ratio of 1:0.5 in patients with colorectal cancer, the method comprising steps (1) and (2): (1) a step for detecting the presence or absence of mutation of the KRAS gene contained in a biological sample collected from the patient, and (2) a step for predicting a high likelihood that the chemotherapy will exhibit an adequate therapeutic effect in that patient, when the result confirmed in step (1) shows that the KRAS gene has mutated.本發明提供一種方法，其係預測對結腸直腸癌患者使用抗腫瘤劑之化學療法之治療效果，該抗腫瘤劑係以1：0.5之莫耳比含有α,α,α-三氟胸苷與5-氯-6-(1-(2-亞胺基吡咯啶基)甲基)尿嘧啶鹽酸鹽者，且該方法包含下述步驟(1)~(2)：步驟(1)：檢測採取自該患者之活體樣本所含KRAS基因有無突變、及步驟(2)：上述步驟(1)中的確認結果，KRAS基因突變時，即預測對該患者的該化學療法會顯示出充分治療效果之可能性高。
