A 4-year-old girl developed a space-occupying lesion in the bladder. This was demonstrated by US after resection of a huge ureterocoele and bilateral re-implantation of ureters at 3 months of age and after three episodes of urinary tract infection. The mass developed outside the field of surgery and was removed endoscopically. Histology revealed a nephrogenic adenoma. US findings have not been emphasised previously. US should be useful in the detection of such lesions and for surveillance of recurrence.