Lewy body-like inclusion (LI) is a cytoplasmic inclusion body deposited in anterior horn cells of patients with amyotrophic lateral sclerosis (ALS), in which there are phosphorylated neurofilament proteins (NFP). Since phosphorylation of NFP occurs normally in the axon, aberrant phosphorylation of NFP should take place in these cell bodies. To clarify the mechanism of LI formation it is crucial to identify the kinase responsible for the phosphorylation. We found cyclin-dependent kinase 5 (cdk5) immunoreactivity in most of LI in an ALS patient. Since cdk5 phosphorylates NFP, cdk5 is a candidate kinase for NFP in LI and might be involved in the cytoskeletal disorganization in LI-containing neurons.