Self-report instruments to assess physical activity are still the most feasible option in many population-wide surveys, and often need to be very short owing to resource constraints. The aim of this study was to test the criterion validity of a single-item physical activity measure using accelerometers and to compare its measurement properties by gender, age group (including older adults) and language region. A validation study was carried out within the second follow-up of a large Swiss cohort study (Swiss Cohort Study on Air Pollution and Lung and Heart Disease in Adults, SAPALDIA, n=208) and included an additional convenient sample (n=110). Participants wore an accelerometer over eight consecutive days and then completed the single-item measure. Spearman's rank-order correlations were used to assess the criterion validity. Physical activity levels were higher in men, younger individuals and those from the German-speaking part of Switzerland. Correlation coefficients for the number of days with at least 30 min of moderate-to-vigorous physical activity according to the single item and different accelerometer activity outcomes ranged from 0.40 to 0.54. Correlations were higher for women, younger individuals and participants from the French-speaking and the Italian-speaking parts. The single-item physical activity measure performed at least as well as other physical activity questionnaires. The differences in criterion validity between sub groups indicate that factors such as gender and age should be taken into account when developing physical activity questionnaires and in future validation studies. Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://group.bmj.com/group/rights-licensing/permissions.