Introduction: In Japan, endometrial cytology is widely performed to evaluate the status of the endometrium in women with suspected endometrial cancer. A new classification system for endometrial cytology has recently been used: the Yokohama system, based on a descriptive reporting format. This study aimed to clarify the triage for patients with atypical endometrial cells of undetermined significance (ATEC-US) when followed by negative endometrial cytology. Methods: We enrolled patients diagnosed with ATEC-US at the Cancer Institute Hospital between January 2016 and December 2017, based on the following inclusion criteria: (1) ATEC-US diagnosed by office endometrial cytology, with or without office endometrial biopsy; (2) follow-up endometrial cytology was performed 3–6 months after initial sampling, with a negative result for malignancy; and (3) no prior history of conservative treatment with progestin for endometrial cancer or atypical endometrial hyperplasia (ATEC-A). Among eligible patients, we analyzed those later diagnosed by endometrial biopsy with ATEC-A or carcinoma. Results: Among 187 patients, 65 met the inclusion criteria. Forty-two patients (64.6%) were observed for more than 24 months. Two patients (3.1%) developed ATEC-A during a median observation time of 26.5 months; the times to diagnosis were 32 months and 22 months. Discussion/Conclusion: No patient developed ATEC-A or worse within 1 year. For patients with ATEC-US, if negative cytology is obtained at the next examination, a close follow-up is not necessary.