The overall frequency of WT1 gene alterations in Wilms tumor is still unclear in Taiwan. Here we conducted molecular genetic analysis of the WT1 gene in Taiwanese patients with Wilms tumor. Polymerase chain reaction and direct sequencing were performed on DNA samples from blood and paraffin-embedded tumor specimens. A constitutional mutation in the WT1 gene was found in one DNA sample from peripheral blood lymphocytes. The remaining DNA samples from peripheral blood lymphocytes and paraffin-embedded tumor specimens were tested negative for both constitutional mutations and somatic mutations. Thus, mutations at other Wilms tumor loci may play an important role in Wilms tumor development.