The prevalence of parkinsonism varies severalfold between different countries in the world. This could be due to an accumulation of risk factors in certain areas. In the present paper we have analyzed the regional distribution of parkinsonism in Sweden in order to see if such areas could be found within one country. The study was based on sales of levodopa in the different counties of Sweden, and the results were standardized for age and quality of health services as indicated by the local infant mortality rates. The mean prevalence of the different Swedish counties was estimated to be in the middle range of known prevalences. There was a 60% variation within Sweden between the different counties, but the regional distribution did not show a systematic pattern. Further analysis of the causes for this variation showed that differences in case ascertainment and in drug policy could well account for the observed regional differences in parkinsonism prevalence.