A pathologic study of 24 cases of primary malignant renal tumors that were examined at the Department of Pathology, Seoul National University Hospital, during period of 5 years from January, 1970 to December, 1974, was made. The results were as follows : 1. The malignant tumors of the kidney constituted 0.9 percent of 2,757 cases of all primary malignant tumors, that were examined during the same period. The mean age of total renal malignant tumors was 38 years : adenocarcinoma 50 years : Wilms’ tumor 6 years : and tumors of renal pelvis 50 years. The renal adenocarcinoma and tumors of renal pelvis were most prevalent after the age of 40 years, while Wilms’ tumor was prevalent before the age of 10 years. The renal tumors affected more males than females with a male to female ratio of 1.4 to 1. Even a single case of benign renal tumor was not noted during the same period. 2. The cardinal symptoms included hematuria, flank pain or discomfort, and palpable abdominal mass. Palpable abdominal mass was the most important and frequent complaints in patients with Wilms’ tumor. Systemic manifestations were observed in 4 cases : hypertension in 1 case. unexplained fever in 2 cases, and polycythemia in 1 case. 3. Twenty-four cases of primary malignant renal tumors were classified histologically into 11 adenocarcinomas(45.8 %), 6 Wilms’ tumors(25.0 %), and 7 carcinomas of renal pelvis (29.2 %). Eleven cases of renal adenocarcinoma included 8 cases of clear cell type(72.7 %), 1 case of granular cell type(9.1 %), and 2 cases of mixed variety(18.2 %). Renal vein invasion was noted in 4 out of 11 cases of renal adenocarcinoma. Epithelial components were observed in all 6 cases of Wilms’ tumor, while striated muscle was observed in only one case.