Fragile site expression in blood lymphocytes of 56 individuals, including 11 Wilms tumor patients, family members, and controls, was studied by 5-fluorodeoxyuridine induction with caffeine enhancement. Significantly elevated expression of fra(11)(p13) was observed in the patient group, compared with controls. One patient clinically diagnosed with the Wilms tumor, aniridia, and genitourinary and retardation (WAGR) syndrome revealed constitutional deletion of 11p13. Expression of fra(11)(p13) in this individual was observed only on the nondeleted homologue. Among the family members studied, only one sibling showed elevated expression of fra(11)(p13).