The unexpected observation of nuclear immunoreactivity for thyroid transcription factor-1 (TTF-1) associated with an apparent lack of nuclear immunoreactivity for Wilms tumor-1 protein (WT1) in the pulmonary metastasis of a morphologically typical case of nephroblastoma affecting a 6.5-year-old male prompted us to examine the expression of these 2 markers (and CD56) in a series of 48 nephroblastomas, 5 adult metanephric adenomas, and 1 pediatric cystic nephroma. TTF-1 was found to be positive in 8 of 48 (16.6%) nephroblastomas and negative in all 5 metanephric adenomas. WT1 was positive in 43 of 48 (89.6%) nephroblastomas and 4 of 5 (80.0%) metanephric adenomas. CD56 was positive in 45 of 47 cases that were so tested (95.74%), but negative in all metanephric adenomas. The single cystic nephroma was TTF-1-negative, WT1-negative, and CD56-positive. The finding of TTF-1 expression in one sixth of nephroblastomas constitutes a potential source of misdiagnosis. The biologic significance of this surprising finding is unclear. It may reflect the embryonal nature of these tumors and may conceivably result-directly or indirectly-in interference with the transcriptional control of target genes and other molecular events in the pathway leading to the development of nephroblastoma.