A case-control analysis of Parkinson's disease and infections in childhood was conducted in a cohort of 50,002 men who attended Harvard College (Cambridge, MA) or the University of Pennsylvania (Philadelphia, PA) between 1916 and 1950 and who were followed in adulthood for morbidity and mortality data. Cases of Parkinson's disease were identified from responses to mailed questionnaires and death certificates through 1978. Four controls from the same population were selected for each case. A reduced risk of Parkinson's disease was associated with most childhood viral infections. The negative association was statistically significant for a history of measles prior to college entrance (exposure odds ratio = 0.53; 95% confidence limits: 0.31, 0.93). The reduced risk of Parkinson's disease among subjects with a positive history of measles in childhood may reflect an adverse effect of measles in adulthood or of subclinical or atypical measles. Furthermore, a negative history of measles, especially if associated with a lack of other common diseases, could be a marker for negative influenza history before 1918 and thus a higher risk of infection during the 1918 influenza epidemic, because of the lack of partial influenza immunity. These data may also suggest a truly protective effect of measles, compatible with some complex interaction between measles virus and the virus of the 1918 influenza epidemic.