This study investigated the validity of the Baecke Questionnaire, the Five City Project Questionnaire, and the Tecumseh Community Health Study Questionnaire in 19 Flemish males, using correlation and multiple stepwise regression analyses. The three questionnaires are commonly used physical activity questionnaires in epidemiological studies. The physical activity level (PAL) as measured with the doubly labelled water method was used as the criterion. The Baecke total activity index showed the highest correlation coefficient with PAL (r = 0.69, p < 0.001). Also the sweat index from the Five City Project Questionnaire, and total daily energy expenditure from the Tecumseh Community Health Study Questionnaire showed significant associations with PAL, respectively 0.57 (p < 0.05) and 0.64 (p < 0.01). Multiple stepwise regression analyses supported the findings from the correlation study. The largest individual contribution in PAL was from the activity index (45%) for the Baecke Questionnaire, the sweat index (29%) for the Five City Project Questionnaire, and total daily energy expenditure (38%) for the Tecumseh Community Health Study Questionnaire. In conclusion, the questionnaires, and certainly the Baecke Questionnaire, can provide valid data about physical activity. Therefore they are useful in epidemiological studies.