Lewy body (LB) in the substantia nigra pars compacta (SNpc) is one of the cardinal pathological features in sporadic Parkinson's disease (sPD). Both pathogeny and pathomechanism of LB have been set forth. However, genetic, postmortem and experimental evidences demonstrate that impaired proteasomes and concomitant LB could result in the concept of process of aggresomes, by which aggregation reaction of abnormal proteins is mainly proposed as molecular crowding of non-fibrillar proteins followed by fibrillation of aggregated proteins, and in which dysfunction of proteasomes, loss of endoplasmic reticulum-associated degradation (ERAD), aggregates, aggresomes and LB are attractive occurrences in sPD. This suggests that dysfunction of proteasome and concomitant formation of LB in sPD is basically associated with cellular process of signal transduction involved in a variety of proteins.