Spinocerebellar ataxia type 1 (SCA1) is a progressive neurodegenerative disorder caused by the expansion of a polyglutamine tract in the ataxin-1 protein. To date, no fundamental treatments for SCA1 have been elucidated. However, some studies have shown that mesenchymal stem cells (MSCs) are partially effective in other genetic mouse models of cerebellar ataxia. In this study, we tested the efficacy of the intrathecal injection of MSCs in the treatment of SCA1 in transgenic (SCA1-Tg) mice. We found that intrathecal injection of only 3 × 10(3) MSCs greatly mitigated the cerebellar neuronal disorganization observed in SCA1 transgenic mice (SCA1-Tg mice). Although the Purkinje cells (PCs) of 24-week-old nontreated SCA1-Tg mice displayed a multilayer arrangement, SCA1-Tg mice at a similar age injected with MSCs displayed monolayer PCs. Furthermore, intrathecal injection of MSCs suppressed the atrophy of PC dendrites in SCA1-Tg mice. Finally, behavioral tests demonstrated that MSCs normalized deficits in motor coordination in SCA1-Tg mice. Future studies should be performed to develop optimal protocols for intrathecal transplantation of MSCs in SCA1 model primates with the aim of developing applications for SCA1 patients.