To evaluate the usefulness of neuronal intranuclear inclusions and neuropil inclusions for the pathological assessment of Huntington's disease (HD), their presence in neocortex was assessed by ubiquitin and N-terminal huntingtin immunohistochemistry in a consecutive series of 195 autopsy brains of individuals with a positive or tentative clinical diagnosis of, or at risk for, HD. The findings were correlated with striatal pathology (n = 190), CAG repeat length (n = 85) and original pathological diagnosis (n = 186). The antibodies detected both these inclusions in 181 patients with HD pathology > or = Vonsattel et al's grade I, five patients lacking striatal tissue for review, and two at-risk individuals with grade 0 and grade I HD pathology, respectively. One patient with HD-like pathology and two patients and four at-risk individuals without HD pathology lacked HD inclusions. In the genetically analyzed cases, the inclusions were exclusively and consistently observed in association with repeat expansion [(CAG)(n) > or = 39, n = 81]. Thirteen inclusion-positive cases, including the grade 0 at-risk individual, had a false negative original pathological diagnosis of HD and four had an unjustly questionable diagnosis. A false positive diagnosis was made in the inclusion-negative case with HD-like pathology. These results indicate that immunohistochemical analysis for HD inclusions facilitates the pathological evaluation of HD and enhances its accuracy.