The genetic epidemiology of late-onset idiopathic Parkinson's disease (PD) and 'parkin-proven' parkinsonism (AR-JP) are limited. The clinical phenotype, prognosis and treatments are similar although PD is prevalent while AR-JP is rare. Molecular genetic and functional analysis suggests the E3 ubiquitin protein ligase activity of parkin, and the ubiquitin-proteosomal pathway, is central to disease pathogenesis. Herein, we compare and contrast PD and AR-JP and discuss the implications of recent data about parkin's genomic organization, regulation and function.