Gene dosage effects for catalase (CAT) were studied in two unrelated patients with an interstitial deletion involving 11p13 to determine precisely the sites of the genes for CAT and the Wilms tumor--aniridia, genitourinary abnormalities, and mental retardation triad (WAGR) in the 11p13 band. Case 1 had the aniridia-Wilms tumor association, and case 2 showed the AGR triad. The karyotypes identified by high resolution banding techniques were 46,XY,del(11)(pter----p13::p11.11----qter) for case 1 and 46,XY,t(2;17)(q23;q25),del(11)(pter----p13::p11.2----qter) for case 2. In both cases, the distal breakpoints of the deleted chromosomes 11 appeared to have occurred on the middle portion of 11p13 (11p1305----p1306). The level of erythrocyte CAT activities in case 1 was reduced (47% of normal), while that in case 2 was normal. The results suggested not only that both the CAT and WAGR should be mapped to chromosome region 11p1305----p1306, but also that in this region the CAT locus is more distally placed than the WAGR locus. Because of the proximity of the two gene loci, assays of erythrocyte CAT may be useful to identify a submicroscopic deletion in some patients with sporadic aniridia and to predict a risk of developing Wilms tumor.