Tumors with anaplasia are said to have unfavorable histology since they show a lesser response to chemotherapy. Three histologic criteria have to be met to classify tumor as having unfavorable histology: 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. It is seen in about 5% of nephroblastomas and is frequently associated with the presence of p53 mutations and resistance to chemotherapy as mentioned earlier. Anaplasia is rarely seen in patients younger than 2 yrs. of age and the prognostic implications do not apply to Stage I tumors. Since anaplasia may be focal, a thorough sampling of the tumor is essential.