Since Lynch syndrome (LS) takes the form of autosomal dominant inheritance, appropriate diagnosis and surveillance may enable early detection and treatment of cancer in the proband or their relatives. Early diagnosis of endometrial carcinoma is important for fertility preservation in women with LS. We report on two patients who were diagnosed with endometrial carcinoma in the early 30s in the middle of infertility treatment. They had MSH2 germline pathogenic variant. Both endometrial carcinomas were found in early stage, but total hysterectomy and postoperative adjuvant chemotherapy were required. It should be considered that their parents had not been diagnosed with LS, despite the treatment of LS-related tumors in our hospital. With appropriate diagnosis of LS and proper surveillance, it is possible that fertility-sparing treatment could have been performed with an earlier diagnosis of endometrial cancer. We realized again that the appropriate diagnosis of the proband could lead a significant impact on medical management, not only for the patient individual but also for the relatives.