Anaplastic Wilms' tumors are commonly believed to be rare forms of progression, driven by p53 mutations, of the more common classical Wilms' tumor or nephroblastoma. Contrary to classical Wilms' tumors, anaplastic tumors traditionally tend to metastasize, to be drug-resistant and to have a poor prognosis. The Bcl-2 gene product protects cells from programmed cell death, and its over-expression has been proposed to be tumorigenic and to mediate resistance to therapy. Because Bcl-2 has been reported to be transcriptionally repressed by p53, using immuno-histochemistry and mRNA analyses, we have examined Bcl-2 expression in a panel of 10 classical and 3 anaplastic nephroblastomas in which the p53 status had been previously analyzed. We found that classical Wilms' tumors expressed significant amounts of Bcl-2 mRNA and protein, whereas anaplastic tumors did not, regardless of p53 mutations. However, because mortality occurred both among patients with classical and among those with anaplastic tumors, which had divergent Bcl-2 expression, analysis of variance failed to demonstrate prognostic Bcl-2 significance. Therefore, we examined the expression of the Bcl-X and Bax genes, which are known to synergize and antagonize Bcl-2, respectively. With the exception of anaplastic tumor W17, the monotony of Bcl-X and Bax mRNA levels did not suggest that the expression of these apoptosisregulating genes could have a role in the prognosis of nephroblastoma. In addition to the standard 2.7-kb Bcl-X L mRNA, W17 expressed a 3.5-kb mRNA species which had the same coding capacity for Bcl-X L as the 2.7-kb mRNA. Western analysis demonstrated that W17 had the highest level of Bcl-X L protein, suggesting that Bcl-X L over-expression could play a part in the development of anaplasia in rare Wilms' tumor cases without affecting prognosis.