Background Wilms' tumor (nephroblastoma) is the most common pediatric kidney cancer. Only one Wilms' tumor gene is known, WT1 at 11p13, which is mutated in 5%-10% of Wilms' tumors. Recently, mutations were reported in WTX at Xq11.1 in Wilms' tumors. This study investigated the mutation proportion, type, and distribution in WTX and WT1 in children with Wilms' tumor. The role of WTX/WT1 in the development of Wilms' tumor, and the relationship between clinical phenotype and genotype, were also studied. Methods Wilms' tumor specimens (blood samples from 70 patients and tumor tissue samples from 52 patients) were used. A long fragment of WTX and 10 exons and intron sequences of WT1 were amplified by polymerase chain reaction (PCR) from extracted genomic DNA and sequenced. A chi-square test compared the difference between the WTX mutation group and the no mutation group. The relationship between the mutations and clinical phenotype was analyzed. Results WTX mutations were found in 5/52 tumor tissues and in 2/70 peripheral blood samples (five cases in total, all point mutations). Two patients had a WTX mutation in both samples. WT1 mutations were found in 2152 tumor tissues and in 4/70 peripheral blood samples (five cases in total, all point mutations). One patient had a WT1 mutation in both samples. Ten cases had WTX or WT1 mutation (19.2% of Wilms' tumors). No overlapping WTX and WT1 mutations were found. No significant differences in clinical parameters were found between patients with and without a WTX mutation. Conclusions WTX mutations occur early in Wilms' tumor development, but at a low proportion. There was no evidence that WTX is the main cause of Wilms' tumor. Clinical parameters of patients with WTX mutations are not related to the mutation, indicating a limited impact of WTX on tumor progression. WTX and WT1 mutations occur independently, suggesting a relationship between their gene products. Chin Med J 2012;125(10):1733-1739