The expression of neurofilament (NF) proteins was examined in the surgical specimen from a 42-year-old woman with Lhermitte-Duclos disease. Hypertrophic granule cell neurons of the dysplastic tissues were reactive with monoclonal antibodies, including antibodies to each of the three human NF subunits. Furthermore, antibodies to dephosphorylation-dependent epitopes on NF proteins stained the cell bodies of hypertrophic granule cells, whereas antibodies to phosphorylation-dependent epitopes stained the enlarged and myelinated axons of the hypertrophic granule cells. Enzymatic dephosphorylation of this tissue abolished axonal staining with phosphorylation-dependent antibodies and uncovered determinants recognized by antibodies to the dephosphorylated state of NF proteins. The NF protein immunoreactivity of hypertrophic granule cells was indistinguishable from that of large, NF-rich neurons in control human cerebellum, suggesting that a normal pattern of expression and phosphorylation of NF proteins occurs in hypertrophic granule cells in Lhermitte-Duclos disease. An increased expression of NF proteins by cerebellar granule cells may account for many of the observed alterations of Lhermitte-Duclos disease, including the hypertrophy of the granule cells and enlargement of their axons, leading to the myelination of parallel fibers within the molecular layer of the cerebellum. Attention should now be directed at the underlying mechanisms which lead to the coordinated up-regulation of the three NF genes and whether or not additional gene products or cell types are altered in Lhermitte-Duclos disease.