The growth characteristics and biological behavior of human tumors transplanted s.c. into two different anatomical regions of nude mice were studied. It was observed that tumors trnasplanted in the anterior lateral thoracic wall grew faster than did tumors transplanted in the posterior aspect of the trunk. Anteriorly growing tumors, in contrast to the posteriorly transplanted ones, were better vascularized, showed less necrosis, invaded the tumor bed, and metastasized to the regional lymph nodes. These findings were independent of their histogenetic and morphological characteristics. It is concluded that regional vascular supply is a key factor influencing the biological behavior of the transplanted tumors and that it may affect tumor response to treatment as well.