Isochromosome 7q is a nonrandom cytogenetic abnormality in Wilms tumor. Two notable cases are described: (1) a case of bilateral Wilms tumor in which only the left-sided tumor contained isochromosome 7q and (2) a case of left-sided Wilms tumor in which the tumor contained isochromosome 7q, in addition to four other chromosomal abnormalities associated with Wilms tumor.