To evaluate if heterogeneity of tissue cancer antigen 125 (CA125) expression is present in epithelial serous adenocarcinomas. Furthermore, to investigate whether there is a correlation between levels of CA125 tissue expression, serum level of CA125, stage, and grade. A total of 10 patients diagnosed with serous ovarian adenocarcinomas were included. Preoperative blood samples were collected to determine serum CA125 levels. Tumor tissue from primary surgery was collected and processed for immunohistochemical analyses. CA125 was expressed in varying degrees in tumor tissues from all patients. Mean tissue CA125 expression for each patient ranged from 36% to 98%. Intrapatient variations in tissue expression ranged from 10% to 90% point. No significant correlations between levels of CA125 tissue expression, serum level of CA125, stage, and grade were found. We found that the tissue expression of CA125 is heterogenic. Although most patients had a high mean expression, it covers a large intrapatient variation in expression. This suggests that if using CA125 as a tissue marker and anti-CA125 (oregovomab) as immunotherapy treatment in future studies, it will be necessary to take heterogeneity into consideration and examine a larger number of biopsies. Therefore, the study demonstrates the need for heterogeneity studies in future translational research.