Wilms' tumor (WT) is the most common malignant renal tumor in childhood. The WT1 gene located at 11p13 was identified in 1990 as a tumor suppressor gene important in the development in WT. The WT1 gene consists of 10 exons, with exons 1 to 6 encoding an N-terminal proline- and glutamine-rich transactivational domain, and exons 7 to 10 encoding a C-terminal zinc-finger domain involved in DNA binding. In China we know little about the frequency and genotype of WT1 mutations in Chinese WT patients. This study aimed to determine the frequency and genotype of WT1 mutations in children with nonsyndromic WT in China. We collected peripheral blood of WT patients treated in Beijing Children's Hospital. Genomic DNA of 54 WT patients was isolated from blood samples. All coding WT1 exons and their flanking intronic sequences were amplified by PCR method. The amplified PCR products from all individuals were then subjected to automatic DNA sequencing. Four different constitutional WT1 mutations were identified in four children. Three mutations are predicted to produce truncated protein. One mutation is missense. Of the four mutations, three had not been reported before. Patient 1 had a 1006 A > T transition in exon 7, which caused (336)Lys to become a stop codon (K336X). DNA sequence analyses in patient 2 indicated the point mutations in exon 9 which was a 1168 C > T substitution and caused (390)Arg to become a stop codon (R390X). It indicated a point mutations in exon 6 in patient 3 which was a 814 G > T substitution and resulted in (272)Glu to become a stop codon (E272X). In patient 4 there was a homozygous mutation in exon 10. The mutation was a 1228 A > G substitution and resulted in (410)Ser to become a Gly codon (S410G). Constitutional WT1 mutations occur at a low frequency (7.4%) in Chinese patients with Wilms' Tumor. It is similar to the results of overseas study. Four WT1 gene mutations were confirmed, three were nonsense, one was missense.