The distribution of metastases in 783 post-mortem examinations for malignant primary tumors has been studied. Mechanical circulatory factors are not very important in determining the sites and extent of metastatic spread. The lungs and liver are the most common sites of blood borne metastates: but when organ weight are taken into account, they fall to the sixth and ninth places respectively. Different tumors, even when their position in the blood stream is the same, have very different metastatic patterns: blood borne metastases through the general arterial circulation of cancers of the lungs and cancers of other organs whose blood is drained by the caval or portal systems, but with metastases to the lungs, are in the ratios of 3:1.5:1 respectively.