The present study examines N-myc and c-myc protein expression with Western blotting and single and double-labeling immunohistochemistry in the hippocampus in Alzheimer disease (AD), the striatum in Huntington disease (HD) and the substantia nigra in Parkinson disease (PD). No modifications in the N-myc and c-myc expression are found in hippocampal neurons in AD, striatal neurons in HD, and pigmented neurons of the substantia nigra in PD. Yet punctate synaptic-like N-myc immunoreactivity, matching enhanced synaptophysin expression, occurs in diffuse plaques, but not in dystrophic neurites of neuritic plaques. In contrast, c-myc immunoreactivity is found in dystrophic neurites, but not in aberrant sproutings of neuritic plaques, as shown by double-labeling immunohistochemistry to c-myc and phosphorylated tau or phosphorylated neurofilament epitopes, and to c-myc and GAP-43, respectively. Strong N-myc and c-myc are observed in reactive astrocytes in AD, HD and PD, as revealed by double-labeling with N-myc or c-myc and GFAP. Finally, no relationship is found between nuclear DNA fragmentation and increased N-myc or c-myc expression in individual cells. These results demonstrate that neuron death in AD, HD and PD is not associated with modifications in the steady-state expression of N-myc and c-myc in individual neurons, and that neurofibrillary degeneration and Lewy body formation are not accompanied by increased immunoreactivity to these transcription factors. Increased N-myc and c-myc expression in reactive astrocytes probably plays a role in reactive astrocytosis in human neurodegenerative disorders.