Fecal occult blood testing is widely used in the clinical screening of colorectal tumors. However, this method has so frequent false-positive results that more accurate screening-strategy should be established. Although the molecular screening using K-ras gene mutation in stools has been attempted to improve the results, the low rate of DNA extraction from stools leaves this measurement under utility value. In this study, we investigated whether or not our applied DNA extraction method from stools could produce enough DNA for the molecular screening of colorectal tumors by K-ras gene mutations in stools. We applied cetyltrimethylammonium bromide (CTAB) solution to improve human DNA extraction from stools and a mutant-allele-sensitive amplification (MASA) method to detect K-ras mutation within codon 12. We were able to confirm the stool DNA by identifying K-ras fragments in all the 20 patients. Tissue K-ras mutation was identified in 4 (2 cancers and 2 adenomas) of 20 patients. Stool K-ras mutations were found in 6 patients, 3 tissue K-ras mutation positive patients (2 cancers and an adenoma) and 3 tissue K-ras mutation negative patients. These results indicate that it is possible to extract enough DNA from human stool samples of all patients with colorectal tumors for K-ras mutation studies. K-ras mutations are more frequently detected in stools than in resected colorectal tumors. This study indicates that K-ras mutation screening in stools for colorectal cancer may include not only a primary colorectal cancer but also precancerous lesions in all parts of a gastrointestinal tract.