A 4-year-old black boy with trisomy 13, a history of frequent urinary tract infections, and a horseshoe kidney with painless gross hematuria was examined. An abdominal mass was detected and surgically resected. Examination of the surgical specimen revealed a Wilms tumor. Given the concurrence of trisomy 13 and Wilms tumor and the presence of another such case in the literature, there may be just cause to suspect a locus on chromosome 13 that affects the probability of developing Wilms tumor. Given the increasingly longer survival of patients with trisomy 13, clinicians may need to be aware of the possibility of renal malignant disease in this population of patients.