Nodular histiocytic aggregate (NHA) of the omentum is a rare benign proliferative process composed predominantly of histiocytes with scattered mesothelial cells. NHA is a differential diagnosis for neoplasms or metastatic tumors in cancer patients. To further clarify this clinical pitfall issue, the authors investigated surgical samples of the greater omentum from 96 patients with gastrointestinal malignancies and 53 patients with gynecologic neoplasms. Visible NHA of greater omentum was identified in 3 patients with ovarian neoplasms (borderline mucinous cystadenoma, low-grade papillary serous cystadenocarcinoma, and juvenile granulosa-cell tumor) but in none of the patients with gastrointestinal malignancies. Similar lesion was also identified on the cell blocks from peritoneal washings in 1 of the 3 patients. Grossly, the lesions formed small yellow-red nodules on the greater omentum, and the NHA lesion was also found diffusely on the surface of the appendix and fallopian tubes in 2 of the 3 patients. Histological study showed that typical NHA changes over an inflammatory background, which may indicate that NHA is a consequence of a chronic inflammatory process of omentum. The predominant infiltration of T lymphocytes in the NHA lesions indicates that the aggregation of histiocytes may be related to the activation of T-cell immunity. This report has first demonstrated visible NHA in the greater omentum of patients with ovarian malignancies, and awareness of this entity should be brought to clinicians to avoid misdiagnosis.