Thyroid transcription factor-1 (TTF-1) is used as an immunohistochemical marker for the identification of the lungs or thyroid gland as the site of origin in patients with metastatic disease and unknown primary tumor. In this study the reliability of anti-TTF-1 was assessed in 65 metastases of the central nervous system (CNS), among which there were also small stereotactic biopsies (n = 22) and poorly preserved specimens. Eight out of nine CNS metastases of patients with known adenocarcinoma of the lungs, as well as seven adenocarcinoma metastases of patients with radiologically detected or anamnestically presumed pulmonary carcinoma, expressed TTF-1 immunohistochemically. One CNS metastasis from a follicular thyroid carcinoma was positive and one from an anaplastic thyroid carcinoma was negative. All CNS metastases from patients with known primary tumors outside the lungs or thyroid gland were negative. TTF-1 is a sensitive (up to 90%) and specific (100%) immunohistochemical marker for CNS metastases of adenocarcinomas of the lungs and also functions reliably on small or stereotactic biopsies and poorly preserved samples.