The evidence implicating oligodendroglia in major mental disorders has grown significantly in the past few years. Microarray analysis revealed altered expression of oligodendroglia-related genes in multiple brain regions from several, clinically diverse groups of subjects with schizophrenia (SZ) as well as subjects with bipolar disorder (BD) and major depressive disorders (MDD), alcoholics and cocaine users. In line with gene expression findings, evidence for ultrastructural changes in white matter and altered oligodendroglia in these disorders were reported in neuroimaging and neuropathological studies. Changes in oligodendroglia-related genes reported in SZ, BD and MDD appear to display considerable similarities (particularly decreased expression of MAG, ERBB, TF, PLP1, MOG, MOBP, MOG), while changes in cocaine abuse and alcoholism are more diverse. Common oligodendroglial abnormalities might indicate aetiological or pathophysiological overlaps between different disorders. The possible mechanisms of oligodendroglial abnormalities may involve functional variations in oligodendroglia-related genes, epigenetic regulation of chromatin, DA system hyperactivity and other mechanisms.