In Reply We appreciate the comments expressed by Darweesh and colleagues and by Macerollo and Chen concerning our article on time trends for Parkinson disease (PD) in Olmsted County, Minnesota.¹ Both groups of authors challenged the attribution of the increasing trend in the incidence of PD to a declining trend in cigarette smoking over time. They also called for an international collaborative research effort to identify risk and protective factors for PD.

The contrasting findings from the Rotterdam Study and from our study suggest that the risk factors for PD may vary across countries, by sex, and over time.² To complicate the landscape, 1 additional study in the United Kingdom and 1 in Taiwan suggested a possible decline in the risk for PD.^3,4 By contrast, a second study in Taiwan showed an increase in the incidence rate between 2002 and 2009.⁵ At this point, we have 5 studies from 4 distinct countries and 3 continents showing results in either direction.