Lynch syndrome (LS) patients have a high risk of developing various tumors. This study aimed to clarify the characteristics of tumors developing in LS patients.This is a retrospective review of 55 LS patients treated at Tokyo Metropolitan Cancer and Infectious Diseases Center Komagome Hospital.The median age at the diagnosis of the first malignant tumor and first LS-related tumor was 44 (range, 19-65) and 44 (range, 24-66) years, respectively. Of the 55 LS patients with developing malignant tumors, 45 (93.8%) developed an LS-related tumor as the first malignant tumor. Colorectal cancer (CRC) developed in 47 patients (85.4%), followed by endometrial cancer (n = 13, 56.5%) in females and gastric cancer (n = 10, 18.1%). In 6 gastric cancer patients, Helicobacter pylori was detected in resected specimens. Twenty-nine patients (52.7%) developed CRC and extra-colonic tumors; of these, 15 patients (48.3%) had mutations in MLH1, 10 (58.8%) in MSH2, and 4 (57.1%) in MSH6. At the age of 50, the cumulative incidence was 50.9% [95% confidence interval (CI), 36.9-63.3%] for CRC, 17.4% (95% CI, 5.2-35.6%) for endometrial cancer, and 5.5% (95% CI, 1.4-13.8%) for gastric cancer. Eight gastric cancer, one breast cancer patient, five bladder cancer patients, and one prostate cancer patient demonstrated loss of expression of the mismatch repair (MMR) protein; patients with thyroid cancer, spindle cell sarcoma, and giant cell tumors did not demonstrate this.Gastric cancer incidence was high in Japanese patients with LS and associated with H. pylori infection. MMR protein deficiency caused the development of malignant tumors in LS patients.