Although choroid plexus papillomas (CPP) and primary choroid plexus carcinomas (CPC) are rare neoplasms of the central nervous system, they have been the subject of a number of immunohistochemical studies. To date, no unique or specific marker for these neoplasms has been found, however. Normal choroid plexus is a major site of transthyretin (TTR) synthesis, and recently this protein has been proposed as a possible specific marker of choroid plexus differentiation in tumors. In this study, we performed immunohistochemistry for TTR on 13 choroid plexus tumors (six CPP and seven CPC) and on 23 carcinomas metastatic to the brain, four of which had a papillary architecture. We also included four ovarian teratomas that contained choroid plexus elements. Two of the CPP had diffuse staining for TTR, while the four others stained only focally. Five of the CPC stained only focally and less intensely than the control, while one case was negative. Only one CPC stained as strongly and diffusely as normal choroid plexus. Two of the papillary and six of the nonpapillary metastases had focal staining similar to that seen in the five focally positive CPC. The choroid plexus elements of the ovarian teratomas stained as strongly as the positive control. These findings indicate that TTR immunoreactivity is not restricted to primary choroid plexus tumors. Furthermore, most choroid plexus carcinomas stain only weakly or not at all. This limits the usefulness of TTR immunohistochemistry in the diagnosis of primary choroid plexus neoplasms and in the distinction of CPC from metastatic carcinoma.