Spinocerebellar ataxia 1 (SCA1) is an inherited neurodegenerative disorder caused by expansion of the polyglutamine stretch in ataxin-1, the SCA1 gene product. Polyglutamine expansion leads to the aggregation of ataxin-1 proteins. Superoxide dismutases (SODs) are involved in the pathogenesis of other aggregate-forming neurodegenerative diseases and are known to localize in the cytoplasm. Here, we show that Cu/Zn-SOD is translocated into the nucleus of HeLa cells in the presence of expanded ataxin-1, whereas Mn-SOD is localized in the cytoplasm: the longer the expansion of polyglutamine, the higher the level of translocation of Cu/Zn-SOD. In addition, the oxidation of intracellular proteins occurs with higher frequency in the presence of mutant ataxin-1 (82Q), suggesting that the functional activity of Cu/Zn-SOD might be decreased by mutant ataxin-1. We demonstrate that mutant ataxin-1-expressing cells encounter mitochondrial dysfunction in the conditions of oxidative stress. Our results suggest that polyglutamine-expanded ataxin-1 increases the levels of reactive oxygen species in HeLa cells.