The gene for red blood cell (RBC) catalase has recently been mapped to 11p13, and a gene dosage effect has been demonstrated for individuals with triplication or deletion of that region. Deletion of the 11pl3 band has also been associated with aniridia, with and without Wilms’ tumor. We studied the RBC catalase levels in individuals without detectable chromosomal abnormalities but with aniridia, Wilms’ tumor, and the combination of aniridia and Wilms’ tumor, to determine whether catalase levels might provide evidence for a sub-microscopic chromosomal deletion in the 11p13 region. All karyotypically normal patients were found to have normal catalase levels.