Alexander disease is a leukodystrophy characterized by the presence of numerous Rosenthal fibers, inclusion bodies in astrocytes. A major component of Rosenthal fibers is alpha B-crystallin, some of which is ubiquitinated. In this report, we show that Alexander central nervous system (CNS) tissues contain elevated messenger RNA and protein levels of both alpha B-crystallin and the related small heat shock protein, hsp27, and that Rosenthal fibers contain hsp27. The alpha B-crystallin and hsp27 polypeptide isoform patterns of Alexander disease CNS are also distinct from those of control samples, suggesting that postranslational modifications may be involved in Rosenthal fiber formation. We advance the hypothesis that Rosenthal fibers may be regarded as stress protein inclusions formed in astrocytes as part of a chronic stress response to an as yet unknown stimulus in the CNS of Alexander patients.