Six of 39 sporadic Wilms tumors had gross homozygous or hemizygous WT1 and WIT1 deletions. Two Wilms tumor‐aniridia‐genitourinary abnormalities‐mental retardation syndrome patients had total hemizygous WT1 and WIT1 deletions in both constitutional and nonsporadic type tumor cells. Four of the 8 tumors with WT1 and WIT1 deletions showed loss of constitutional heterozygosity (LOH) for markers limited to the 11p13 region. Seven of 19 Wilms tumors with neither WT1 nor WIT1 deletions also had LOH on 11p; 4 in the 11p15–11p13 region, one in the 11p15 and possibly also 11p13 regions, and two solely in the 11lp15 region. Thus, 15 of the 41 Wilms tumors (37%) had WT1 and WIT1 deletions or LOH on 11p, and only 2 of the 27 tumors whose nonneoplastic normal tissues were available for study showed LOH limited to the 11p15 region. None of the 7 non‐Wilms childhood renal tumors showed WT1 or WIT1 deletions, or LOH on 11p. These data suggest that Japanese Wilms tumors may be characterized by a higher incidence of the gross WT1 deletion and a lower incidence of LOH limited to the 11p15 region than the Caucasian counterparts. These moleculargenetic features may be contributing to the lower incidence of Wilms tumors in Japanese children than in Caucasian ones.