Studies of huntingtin localization in human post–mortem brain offer insights and a framework for basic experiments in the pathogenesis of Huntington'sdisease. In neurons of cortex and striatum, we identified changes in the cytoplasmic localization of huntingtin including a marked perinuclear accumulation of huntingtin and formation of multivesicular bodies, changes conceivably pointing to an altered handling of huntingtin in neurons. In Huntington'sdisease, huntingtin also accumulates in aberrant subcellular compartments such as nuclear and neuritic aggregates co–localized with ubiquitin. The site of protein aggregation is polyglutamine–dependent, both in juvenile–onset patients having more aggregates in the nucleus and in adult–onset patients presenting more neuritic aggregates. Studies in vitro reveal that the genesis of these aggregates and cell death are tied to cleavage of mutant huntingtin. However, we found that the aggregation of mutant huntingtin can be dissociated from the extent of cell death. Thus properties of mutant huntingtin more subtle than its aggregation, such as its proteolysis and protein interactions that affect vesicle trafficking and nuclear transport, might suffice to cause neurodegeneration in the striatum and cortex. We propose that mutant huntingtin engages multiple pathogenic pathways leading to neuronal death.