We examined the clinicopathological features of 4 patients with Lynch syndrome (including those with strongly suspected Lynch syndrome) who had undergone chemotherapy as part of the treatment for colon cancer. The study population included 3 men and 1 woman. The median age at diagnosis was 72 years (range, 24-40 years). Two patients relapsed and 2 patients had Stage IV cancer. All the patients had primary right-sided colon cancer. On histopathological examination, well differentiated adenocarcinoma, moderately differentiated adenocarcinoma, and poorly differentiated adenocarcinoma were observed in 1, 2, and 1 patients, respectively. Components of mucinous adenocarcinoma or poorly differentiated adenocarcinoma were observed in all patients. Microsatellite instability (MSI) testing revealed high MSI (MSI-H) in all patients. Immunostaining results indicated loss of expression of the MSH2/MSH6 proteins at the tumor site in all patients. With regard to chemotherapy, a 5-fluorouracil-based regimen was selected as the first-line of therapy. Although 1 patient experienced long-term survival (alive at 48 months after surgery), the other 3 patients died of cancer within 9 months. The median overall survival was 8.5 months. Active assessment and accumulation of such cases is essential for the establishment of appropriate and effective treatments for colon cancer in patients with Lynch syndrome.