Objectives: To assess the difficulties associated with diagnosing endometrial stromal tumors (ESTs) on endometrial biopsy. Methods: We examined 25 endometrial biopsy specimens from 19 consecutive women diagnosed with either endometrial stromal nodule (n = 3) or endometrial stromal sarcoma (n = 16). Results: Rereview of the biopsy specimens revealed a stromal fragment suspicious for an EST in 16, of which eight had received a benign diagnosis on initial review. Most ESTs had an aglandular stromal fragment that was 5 mm or larger. Stromal fragments of this size were not encountered in the control material. Problematic areas included highly cellular leiomyoma and a lack of attention to the stromal compartment. Conclusions: Most endometrial stromal tumors present with large aglandular stromal fragments (≥5 mm). These fragments are large enough that difficulties in diagnosis appear to be due to a lack of attention to the stromal compartment.