A 5-DAY-OLD boy was seen for poor feeding, nonbilious vomiting, lethargy, and decreased urine output. The history was notable for an unremarkable pregnancy with normal findings from 16-week fetal ultrasound. Findings from initial physical examination revealed a large, nontender right flank mass. Results of subsequent laboratory analysis revealed acute renal failure, metabolic acidosis, hyperkalemia, azotemia, and hematuria. Anemia and thrombocytopenia were also present. Renal ultrasonography was performed, and results of a subsequent computed tomographic scan of the abdomen are shown in Figure 1 and Figure 2.