We report six cases of hyperplastic mesothelial cells located in the sinuses of lymph nodes. All patients but one had a concurrent serosal fluid collection (two pericardial, two pleural, one abdominal) at the time of the lymph node biopsy. All effusions cleared with treatment of the underlying disorder, which included lymphoproliferative processes, congestive heart failure, and inflammatory diseases (Dressler syndrome, vasculitis, and glomerulonephritis). Four cases were associated with vascular prominence of the involved nodal sinuses, a feature that may reflect the cause of the underlying effusion or support the transient persistence of benign mesothelial cells in lymph nodes. Two cases were characterized by distention of the nodal sinuses by sheets of mitotically active mesothelial cells. The differential diagnosis includes metastatic carcinoma, keratin-positive dendritic cells native to lymph nodes, and metastatic malignant mesothelioma. Because the latter shares both clinical and morphological features with cases of benign mesothelial cells in lymph nodes, we believe that this distinction may not always be possible in a given biopsy specimen and therefore that careful clinical follow-up is required in such cases.