The inability to purify and culture astrocytes has long hindered studies of their function. Whereas astrocyte progenitor cells can be cultured from neonatal brain, culture of mature astrocytes from postnatal brain has not been possible. Here, we report a new method to prospectively purify astrocytes by immunopanning. These astrocytes undergo apoptosis in culture, but vascular cells and HBEGF promote their survival in serum-free culture. We found that some developing astrocytes normally undergo apoptosis in vivo and that the vast majority of astrocytes contact blood vessels, suggesting that astrocytes are matched to blood vessels by competing for vascular-derived trophic factors such as HBEGF. Compared to traditional astrocyte cultures, the gene profiles of the cultured purified postnatal astrocytes much more closely resemble those of in vivo astrocytes. Although these astrocytes strongly promote synapse formation and function, they do not secrete glutamate in response to stimulation.