Cylooxygenase-2 (Cox-2) inhibitors have increasingly become therapeutic alternatives in some Cox-2-overexpressing neoplasms. As the treatment eligibility for these drugs hinges on Cox-2 expression, Cox-2 immunostaining has recently been widely examined in several malignant neoplasms. However, data on the expression of Cox-2 in Wilms' tumor (WT) are limited. In this study, we examined Cox-2 expression in 40 examples of WT to identify the prognostic impact, to evaluate the effects on tumorigenesis, and to answer the question of whether neoplasms with Cox-2 expression could benefit from treatment with specific Cox-2 inhibitors. Sections from paraffin-embedded tumor samples were immunostained by a standard ABC technique using Cox-2 mouse monoclonal antibody. As in other rare examples reported in the literature, Cox-2 immunoreactivity was analyzed and correlated with histological features and the staging of neoplasms. However, in contrast to other studies, we also evaluated the relation of Cox-2 positivity to age, sex, and survival of patients. The results of this study demonstrated that Cox-2 was ubiquitously expressed in all cases of WT and their neovasculature, independently of the type of neoplasm (tumors with a favorable or unfavorable histology), tissues which constitute the neoplasm (blastemal, mesenchymal and epithelial, heterologous or non-heterologous elements), patient age, sex, or stage of development and survival rate. Thus, Cox-2 inhibitors could be used for treating all cases of WT. Further studies, including molecular investigations, would be useful to confirm our hypotheses.