Background—The purpose of this paper is to evaluate self reported physical activity of young adults using one-week and one-year recall measures with an accelerometer as the criterion measure. Methods—Participants were a subsample (N=121, 24±1.7 yrs) from a large longitudinal cohort study. Participants completed a detailed one-year physical activity recall, wore an accelerometer for one week and then completed a brief one-week physical activity recall when they returned the accelerometer. Results—Mean values for moderate-to-vigorous physical activity (MVPA) from the three instruments were 3.2, 2.2, and 13.7 hours/wk for the accelerometer, one-week recall, and one-year recall, respectively (all different from each other, p<0.001). Spearman correlations for moderate, vigorous, and MVPA between the accelerometer and the one-week recall (0.30, 0.50, and 0.40, respectively) and the one-year recall (0.31, 0.42, and 0.44, respectively) demonstrated adequate validity. Conclusions—Both recall instruments may be used for ranking physical activity at the group level. At the individual level, the one-week recall performed much better in terms of absolute value of physical activity. The one-year recall overestimated total physical activity but additional research is needed to fully test its validity.