BACKGROUND: Although endoscopic resection is increasingly performed to treat submucosal invasive colorectal cancer (T1CRC), approximately 10% are at risk of lymph node metastasis. The Japanese Society for Cancer of the Colon and Rectum guideline indicates that the following risk factors should be considered when deciding whether to perform additional surgical resection with lymph node dissection: depth of T1 invasion, lymphovascular invasion, poor histological grade, and budding grade 2/3. However, there is little information about the prognosis of T1CRC patients, or factors to consider when deciding subsequent treatment of high-risk T1CRC. METHODS: This retrospective mixed method study was conducted using electronic medical records at Kyoto University Hospital between February 2005 and February 2015. Participants were T1CRC patients at risk of lymph node metastasis with at least one of the above four risk factors. They were assigned either careful follow-up (FU) or additional surgery (AS) through shared decision-making. To identify factors affecting decision-making in the FU group, we performed qualitative content analysis of electronic medical records. The prognosis of the groups was compared using the Kaplan-Meier method and the log-rank test. RESULTS: Of 161 T1CRC patients, 18 were included in the FU group and 19 in the AS group. The median follow-up time was 39.5 (range 23-126) months for the FU group and 62 (range 22-141) months for the AS group. Factors considered in selecting FU were advanced age, comorbidities, the sole presence of the "depth" risk factor, and lower rectal cancer. For AS, the risk factors cited in the guideline were considered. There was one recurrent case in each group during the research period. There were no significant differences in overall survival, cause-specific survival, or recurrence-free survival between the groups. CONCLUSIONS: Age, comorbidities, and lower-rectal cancer location were considered in deciding posttreatment strategy among high-risk T1CRC patients, alongside with positive vertical margin, depth, lymphovascular invasion, poor histologic grade, and budding. During the research period, there was no prognostic difference between the FU and AS groups.