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. In its classic microscopic appearance, Wilms tumour is triphasic and consists of variable proportions of blastema, stroma, and epithelial cells. One component may dominate over others in a given case, resulting in biphasic and monophasic variants. The blastemal component (shown here) of a Wilms tumour has the prototypical appearance of a small round blue cell tumour. It is highly cellular and consists of undifferentiated small round-to-oval cells with hyperchromatic nuclei and scant cytoplasm. The cells may be arranged in diffuse sheets, serpentine, or nodular patterns. Mitotic activity is generally brisk. Apoptosis and foci of necrosis may be seen within blastema.