Primary brain tumors invade and metastasize within the central nervous system (CNS), but rarely extraaxially. A model explaining this behavior is presented. In non-CNS tumors metastases occur since subpopulations of cells capable of invasion and metastases are selected by the tissular environment, and in particular by the connective stroma, which opposes invasion. The CNS lacks a connective stroma, therefore a primary tumor can grow easily in it, but subpopulations capable of metastasization are not selected. Implications of the explanation are: 1) metastasization is largely based on the ability to invade the connective stroma, 2) paradoxically, a less favourable host tissue environment can result in a more aggressive tumoral behavior.