The average age incidence was 4.2 years; the youngest patient was 9 months of age and the oldest one was 23 years of age. Three fourths of the patients were children under three years of age. Seven were males and five were females. The incidence of Wilms tumor among all renal cancers was 57 per cent and this is much higher than that reported in the foreign literature. This high incidence may be attributed to a much higher incidence of this neoplasm among Korean infants and children.

The most prominent clinical finding was abdominal swelling and a palpable mass, but fever, hypertension, irritability, hematuria and cough were other common symptoms. Hypertension was encountered in 89 per cent of the cases. The common laboratory findings were anemia, leukocytosis, increased erythrocyte sedimentation rate and proteinuria.

Gross and microscopic findings are described. All specimens were much 1arger than the normal kidney. The largest specimen, kidney plus tumor, weighed 1,800 gm. The tumors were composed of well formed tubules. solid islands of dark staining undifferentiated cells presumably of mesoblastic origin and various elements of stromal cells. The proportion of each element varied markedly from case to case and also in different parts of the same tumor. Nests of squamous cells, some of them forming pearls, and cystic structures identical with epidermal cysts were observed in three of our 12 cases, but unequivocal striated muscle cells as well as bone or cartilage were found in none of our cases. Some nests of squamous cells observed in tubules suggested that they were derived from metaplasia of the lining epithelium.