Patients with differentiated thyroid cancer and extensive local invasion have a generally poor prognosis. We present a patient with a large papillary thyroid cancer and apparent substernal extension with aggressive local invasion. Sternotomy with an intent to debulk the tumor revealed inoperable disease, but open biopsy unexpectedly revealed a large B-cell mediastinal lymphoma completely enveloping the great vessels and bronchial tree and juxtaposed to a large but noninvasive papillary thyroid cancer. This finding allowed proper curative therapy for what otherwise might have been considered inoperable thyroid cancer with anaplastic transformation. Seven years following diagnosis the patient remains disease free from both the thyroid cancer and mediastinal lymphoma.