The distributions of class III alcohol dehydrogenase (ADH), a glutathione-dependent formaldehyde dehydrogenase, and class I ADH in the human brain were examined immunohistochemically. The most intense immunostaining of class III ADH was observed in the dendrites and cytoplasm of cerebellar Purkinje cells. Scattered cerebral cortical neurons in layers IV and V, and some hippocampal pyramidal neurons were also immunopositive. The neuronal distribution of class III ADH resembled that of the vulnerable neurons in patients with hypoxic encephalopathy, which in view of the intense staining in the Purkinje cells, raises the possibility that this enzyme contributes to the hypoxia and cerebellar degeneration suffered by chronic alcoholics. Perivascular and subependymal astrocytes, which contribute to the maintenance of the cerebral cellular milieu and isolate the brain from the systemic circulation and cerebrospinal fluid, were also class III ADH positive. As the substrates of this enzyme include intrinsic toxic formaldehyde, inflammatory intermediate of 20-hydroxy-leukoteiene B4, and possibly ethanol, the distribution of class III ADH immunostaining indicates this enzyme contributes to the defence of the brain against degenerative processes. The finding that, unlike ependymal cells, subependymal astrocytes were class III ADH positive, suggests this enzyme may be useful for differentiating astrocytes and ependymal cells.