Neuropathological investigations performed on autopsied brain and spinal cords from 11 patients showed that spinocerebellar ataxia type 1 (SCA-1) can be distinguished from autosomal dominant spinocerebellar ataxia linked to SCA-2 and-3 loci on chromosomes 12 and 14, spinopontine, and the multisystem atrophies. The major diagnostic criteria were: absence of significant pars compacta nigral and locus coeruleus lesions, severe degeneration of olivocerebellar and dentatorubral pathways, extensive loss of Purkinje cells with partial sparing of flocculonodular lobes, severe atrophy of specific cranial nerve nuclei, mostly the third and 12th, extensive loss of motor neurons in anterior horns and Clarke's columns, and lack of oligodendroglial or neuronal cytoplasmic cytoskeletal inclusions. None of the brain displayed any significant immunoreactivity for the amyloid precursor protein or β-amyloid peptide throughout the cortex. In conclusion, the type and neuroanatomical distribution of structural lesions were similarly reproduced in all probands at the end stage of SCA-1, to the point that they appeared to constitute a unique phenotype.