The main goal of the present study is to validate the \u201cFour by One-Day Physical Activity Questionnaire\u201d in a sample of 37 children (13.6?\u20091.13) years, by comparing self-reported average daily energy use in moderate and vigorous physical activities against the steps recorded by pedometers and with monitored minutes in different cardiac frequency limits. The findings in test-retest range from 0.84 to 0.93 (p<\/em><0.05). In the present study, the Four by One-Day Physical Activity Questionnaire was modestly but significantly (p<\/em>=0.01) correlated with minutes above a threshold of 50% of heart rate reserve (r<\/em>=0.578), with minutes above a threshold of 60% of maximum heart rate (r<\/em>=0.596), with minutes above a threshold of 140 beat\/min (r<\/em>=0.625) and with average daily steps (r<\/em>=0.672). The Spanish version of the \u201cFour by One-Day Physical Activity Questionnaire\u201d had similar reliability and validity to the original version. The\u00a0results obtained show that this questionnaire can be a valid and reliable means of finding out the PA levels of Spanish adolescents.<\/p>\n\n