Cytogenetic and molecular analysis of Wilms tumors have led to the identification of two regions on the short arm of chromosome 11 (11p13 and 11p15) involved in tumor development. Recent studies have provided evidence that an additional locus on 16q is also involved. Further molecular testing may reveal additional loci associated with the development or progression of this tumor. Reports of single chromosome abnormalities in tumors generally pinpoint regions of interest that may be involved in the etiology of the tumor. We present an additional case of Wilms tumor with an isochromosome 7q as the sole cytogenetic change, resulting in loss of 7p and gain of 7q material.