Seventy-five cases of primary retroperitoneal tumors observed at the Istituto Nazionale Tumori of Milan during the period 1932 to 1973, 56 of which had a histopathologic classification (51 malignant and 5 benign), are reported. By examination of the diagnostic and therapeutic criteria employed and the survival data (9 of the 51 malignant cases were alive at 3 years, and 3 of these were alive at 5 years), the authors point out the difficulty of an early diagnosis and, therefore, of a radical surgery, the need for a programmed therapy, and the importance of the angiographic examination in the diagnostic strategy.