Objective: Histopathological variation has been demonstrated in grade 3 endometrioid adenocarcinomas. We attempted to evaluate the clinicopathological features of grade 3 tumors by endometrial cytological features using a scoring system. Study Design: Twenty-one endometrial cytological samples were evaluated using 5 cytological features: rates of cluster formation in tumor cells; nuclear pleomorphism; nuclear dimension; size of nucleoli, and chromatin structure and distribution. The relationships between cytological scores and clinicopathological factors or prognosis were investigated. Results: The median cytological score was 6 (range 4-14); therefore, samples with scores of 4-5 were defined as having low scores, while those with scores of 6-14 were defined as high scores. The accuracy of the cytological diagnosis for grade 3 tumors in the high score group (8/10 patients, 80.0%) was significantly higher than that of the low score group (2/11 patients, 18.2%; p = 0.009). Significant relationships between cytological scores and lymph node metastases or positive peritoneal cytology were observed (p = 0.03 and 0.035, respectively). The overall survival rate was significantly worse in the high score group (30.0%) than the low score group (88.9%; p = 0.02). Conclusions: Grade 3 endometrioid adenocarcinomas varied in cytological features; according to the scoring system used, high scores were associated with worse clinicopathological factors and poorer prognosis than low scores.