The immune cytokine interferon-gamma (IFN-gamma) plays a crucial role in immune-mediated demyelinating diseases such as multiple sclerosis and experimental autoimmune encephalomyelitis (EAE). Our previous studies have shown that enforced expression of IFN-gamma in the central nervous system (CNS) inhibits developmental myelination or remyelination in EAE demyelinated lesions. Although many of the cellular actions of IFN-gamma result from its activation of the signal transducer and activator of transcription 1 (STAT1) pathway, recent studies have shown that STAT1-independent pathways regulate some facets of IFN-gamma biology. In this study, we dissected the role ofSTAT1-dependent and STAT1-independent pathways in IFN-gamma-induced hypomyelination using a genetic approach. We found that the induction of STAT1-dependent, IFN-gamma-responsive genes in response to this cytokine was abolished in the CNS of STAT1 null mice. Moreover, STAT1 deletion diminished oligodendrocyte loss, reduction of myelinated axons, and the inflammatory response in the CNS of transgenic mice that ectopically expressed IFN-gamma in the CNS. Nevertheless, IFN-gamma-induced reduction of myelin sheath thickness in the CNS of these mice was not altered by STAT1 deletion. Collectively, these data demonstrate that both STAT1-dependent and STAT1-independent pathways are involved in the detrimental effects of IFN-gamma on the myelination process.