The validity is explored of a short and simple questionnaire to differentiate populations into physical active and physical inactive persons according to recent guidelines on the required amount of physical activity (how many days a week they have physical activities for minimal 30 minutes a day). 213 adults and 168 children completed this questionnaire as well as a more detailed validated questionnaire about their physical activities. Mean physical activity score as measured with the comprehensive questionnaire was higher for those who were classified as 'active' in the simple questionnaire. However, a prediction of the activity-score of the comprehensive questionnaire on the basis of the simple questionnaire resulted in a relatively high rate of misclassification (23 to 41%). Due to the fact that there is no gold standard, definitive conclusions are difficult to be drawn.