A long preclinical or asymptomatic period may occur in Parkinson's disease (PD). Many long-latency parkinsonian syndromes exist. The presence of early-life risk factors is consistent with a long prodromal period. Marked degeneration of the substantia nigra and loss of striatal dopamine are necessary before clinical symptoms develop. Lewy bodies, the histological hallmark of PD, occur in 10% of normal individuals over age 50. Clinical symptoms develop slowly and are often intermittent in early PD. Nonmotor signs, eg, depression or sensory changes, often precede motor signs by many years. Reduction of striatal dopamine can be detected with PET in "at-risk" asymptomatic individuals. Individual sensitivity to drug-induced parkinsonism also suggests a preclinical state. Biologic markers may eventually be able to detect preclinical PD.