The interobserver diagnostic reproducibility for epithelial ovarian neoplasia was studied. The histologic sections of 68 ovarian tumors from 34 patients were independently reviewed by two pathologists from different institutions, without knowledge of the clinical course. Each observer rendered 68 diagnoses. The interobserver agreement rate for histologic type was 60% and for histologic grade 66%. In the 23 instances in which a discrepancy in grade occurred, 5 (23%) were disagreements in the diagnosis of borderline versus malignant tumors. On analysis of the diagnostic variability with regard to histologic type, one observer (A) classified 60% of the tumors as undifferentiated, while the other observer (B) classified 59% of the tumors as serous. In an attempt to understand the reasons for the diagnostic disagreements, the observers were asked to simultaneously reexamine the material. This occurred 6 months after the initial review and they were unaware of their original diagnoses. The diagnostic differences with regard to histologic type were mainly due to (1) tumor cell heterogeneity, and (2) difficulty in discriminating between serous and undifferentiated tumors. The differences in grade were largely related to the use of different criteria. Observer A used mitotic counts while observer B used glandular pattern and its replacement by solid sheets of epithelium. Observer B consistently assigned a higher grade to the tumors. In making therapeutic decisions, clinicians should be aware of the interobserver diagnostic variability and the reasons for this variability.