Supradiaphragmatic lymhadenopathy is extremely rare in patients with a serous borderline ovarian tumor (BOT), and clinically difficult to recognize. We describe 3 cases of serous BOT that primarily presented with arm thrombosis due to supradiaphragmatic lymphadenopathy. In all the 3 cases, fine needle aspiration cytology initially indicated metastatic adenocarcinoma. The primary tumor was not immediately apparent, and multiple diagnostic examinations had to be done before the definitive diagnosis of serous BOT, International Federation of Gynecology and Obstetrics stage IV could be made. In the meanwhile, erroneous therapies had been given in 1 case. After surgical removal of the adnexal masses and full surgical staging, all the 3 patients remained free of disease after a follow-up period of 48 to 84 months. In conclusion, supradiaphragmatic lymph node involvement can be present in patients with serous BOTs, and can even be the presenting symptom. When fine needle aspiration cytology of such a lymph node is compatible with adenocarcinoma of unknown primary, serous BOT should be included in the differential diagnosis and pelvic examination should be performed.