Neuronal intranuclear inclusion disease (NIID) is a rare neurodegenerative disorder characterized by progressive ataxia and neuronal nuclear inclusions (NIs), similar to the inclusions found in expanded CAG repeat diseases. NIID may be familial or sporadic. The cause of familial NIID is poorly understood, as no CAG expansion has been detected. We examined three cases, from two unrelated families, who had autosomal dominant NIID but normal CAG repeats in genes involved in polyglutamine neurodegenerative diseases. We found that NIs in all three cases were intensely immunopositive for SUMO-1, a protein which covalently conjugates to other proteins and targets them to the nuclear regions (nuclear bodies) responsible for nuclear proteasomal degradation. Electron microscopy demonstrated that SUMO-1 was located on the 10-nm fibrils of NIs. In cultured PC12 cells, we found that inhibition of proteasome function by specific inhibitors resulted in the appearance of SUMO-1-immunopositive nuclear inclusions. Our study suggests that recruitment of SUMO-1 modified proteins into insoluble nuclear inclusions and proteasomal dysfunction may be involved in the pathogenesis of NIs in familial NIID cases.