The prevalence of idiopathic Parkinson's Disease (IPD) in Dunedin, New Zealand on 31st July 1990 was 110.4/100,000. When corrected to a standard population based on the 1960 U.S. census, the prevalence fell to 76.0/100,000 due to changes in the age structure of the population. The corrected prevalence in Wellington (another New Zealand city), in 1962 was 99.6 (before the introduction of levodopa), and in Aberdeen, Scotland in 1984 was 102.7. The principal difference was fewer people under 65 years of age in our study. Case finding methods and diagnostic criteria were similar in all three studies, and case ascertainment was adequate. Under representation of younger people could be due to either a lower incidence rate or poorer survival due to treatment with high doses of levodopa compounds. Prospective research is required to explain our findings.