We have examined with 19 tumor cell lines the discrete roles that vascular anatomy and tumor-cell-organ-affinity play in the development of metastases and their distribution among organs. Spontaneous metastases of B16-G3.26 melanoma cells from a primary tumor growing in the foot pad of mice, or experimental metastases 21 days after intravenous tumor-cell injection resulted in tumor colonies only in the lungs. In contrast, when the lung microvasculature was bypassed, and the same cells given by systemic intra-arterial (s.i.a.) injection, large tumor colonies developed selectively in the ovaries, adrenal glands and bones, but rarely in the lungs. When animals injected i.v. were allowed to live with lung metastases for a long period of time, small tumor colonies began to develop in extra-pulmonary organs with a distribution identical to that seen after s.i.a. injection. Seven murine tumor cell lines (previously characterized by their ability to colonize primarily the lungs after i.v. injection) and 7 of the 8 studied human tumor cell lines colonized different specific extra-pulmonary organs after s.i.a. injection, frequently producing metastatic syndromes commonly described in patients with cancer, but rarely seen in animal models of metastasis. These results suggest that metastatic cells, even those capable of colonizing specific organs, do not freely circulate in the blood stream and lodge in specific tissues. In contrast, the cells must establish a vascular route of access to the target organ, e.g., through the systemic circulation from metastatic tumors in the lungs. Two cell lines considered to be tumorigenic but non-metastatic failed to colonize the lungs or extra-pulmonary organs after i.v. injection, but readily colonized specific organs after s.i.a. injection. Thus, tumor cells considered to be non-metastatic may be indeed metastatic if they are provided with vascular access to an organ more congenial to their growth requirements.