Despite the major finding of a genetic defect being responsible for the Parkinson's disease (PD) phenotype in some kindreds with dominantly transmitted PD, the role of inheritance in the cause of the more widespread sporadic form of the disease is still unclear. Twin studies are a classic tool for assessing the influence of hereditary factors in diseases; however, the application of this approach to late-onset illnesses, like PD, poses some problems because of the identification of subclinical cases. In the present longitudinal study we have used [18F]dopa and positron emission tomography to study dopaminergic function in twin pairs at baseline clinically discordant for PD. At baseline, the concordance for subclinical striatal dopaminergic dysfunction was found to be significantly higher in 18 monozygotic than in 16 dizygotic twin pairs (55% vs 18%, respectively). The asymptomatic monozygotic cotwins all showed progressive loss of dopaminergic function over 7 years and 4 developed clinical PD. None of the dizygotic twin pairs became clinically concordant. At follow-up, the combined concordance levels for subclinical dopaminergic dysfunction and clinical PD were 75% in the 12 monozygotic and 22% in the 9 dizygotic twin pairs evaluated twice. Our findings suggest a substantial role for inheritance in sporadic PD.