Nephroblastoma, light micrograph. Nephroblastoma, or Wilms tumour, is the fourth most common type of childhood cancer and the most common type of kidney cancer in children. The peak incidence is seen between 2 and 5 years of age. It is only rarely seen as a congenital neoplasm. They are usually solitary, large, and sharply demarcated tumours. About 7% are multicentric and 5% of cases are bilateral. Wilms tumours with anaplasia are said to have unfavourable histology since they show a lesser response to chemotherapy. Microscopically, they show 1) nuclei enlarged to at least 3 times the size of adjacent nuclei of the same cell type; 2) marked hyperchromasia of the enlarged nuclei; and 3) multipolar mitotic figures. Unfavourable histology is seen in about 5% of Wilms tumours and is frequently associated with the presence of p53 mutations and resistance to chemotherapy.