Primary ovarian lymphomas are rare, but can potentially evoke diagnostic problems. We present a case of ovarian lymphoma, in which an ambiguous intraoperative pathologic report led to overtreatment (unnecessary surgery). A 73-yr-old woman with fatigue and low-grade fever was diagnosed as having a left ovarian tumor by imaging modalities. Exploratory laparotomy was carried out to confirm the diagnosis. The frozen tissue sections of the ovarian tumor showed condensed proliferation of atypical round cells accompanied with a few small lymphocytes. The pathologists could not determine whether this tumor was a lymphoma or another malignancy (eg, dysgerminoma). Hence, they reported it to gynecologists who operated as simply a malignant tumor in order to evade misdiagnosis. On the basis of the inconclusive pathologic report, the gynecologists decided to change the planned laparotomy to total hysterectomy with bilateral salpingo-oophorectomy and pelvic lymph node dissection. A postoperative paraffin section-based pathologic diagnosis was diffuse large B-cell lymphoma of the ovary, which basically does not require surgical treatments. Subsequently, chemotherapy for B-cell lymphoma was initiated, and no lymphoma recurrence has been reported to date. A more robust preoperative discussion between the gynecologists and the pathologists might have avoided the overtreatment.