Background: Endometrial clear cell carcinoma (ECCC) is a rare variant of endometrial cancer with an unfavorable prognosis. The aim of this study was to determine the relationships, if any, between preoperative serum cancer antigen 125 (CA-125) levels, the last Papanicolaou (Pap) smear, the results of endometrial biopsy and tumor histology, and disease-stage according to permanent histopathology. Materials and Methods: The data of 26 women with ECCC were collected from their medical records to compare demographics, preoperative serum CA-125 levels, Pap test results, histological diagnosis of the endometrial biopsy, and tumor histology, as well as the disease stage according to permanent histopathology. Results: The mean age at diagnosis was 64.0 years. There were 17 (65.3%) women with pure ECCC and 8 (30.7%) women with a mixed histology. Stage III-IV disease was diagnosed in 12 (46.2%) women and occurred more frequently in pure ECCC (n = 11, 91.7%). CA-125 values were significantly higher in advanced stage disease. Cytological evaluation indicated the presence of 71.4% (10/14) cytological abnormalities in pure ECCC. The overall sensitivity of endometrial sampling for the detection of malignancy was 92.3% (24/26), whereas the accurate diagnosis of ECCC was only 34.7% (8/23) with the Pipelle sampler. Conclusion: Pap test abnormalities are frequent in ECCC. Although it is less accurate in the diagnosis of ECCC than in the detection of malignancy, endometrial sampling is still the main procedure for the diagnosis of ECCC. Higher preoperative CA-125 concentrations imply the presence of advanced stage ECCC.