Background: Self-reported confidence ratings have been used in other research disciplines as a tool to assess data quality, and may be useful in epidemiologic studies. Methods: We examined whether self-reported confidence in recall of physical activity was a predictor of the validity and retest reliability of physical activity measures from the European Prospective Investigation into Cancer and Nutrition (EPIC) past-year questionnaire and the International Physical Activity Questionnaire (IPAQ) last-7-day questionnaire.During 2005-2006 in Sydney, Australia, 97 men and 80 women completed both questionnaires at baseline and at 10 months and wore an accelerometer as an objective comparison measure for three 7-day periods during the same timeframe. Participants rated their confidence in recalling physical activity for each question using a 5-point scale and were dichotomized at the median confidence value. Results: Participants in the high-confidence group had higher validity and repeatability coefficients than those in the low-confidence group for most comparisons. The differences were most apparent for validity of IPAQ moderate activity: Spearman correlation ρ = 0.34 (95% confidence interval [CI] = 0.08 to 0.55) and 0.01 (-0.17 to 0.20) for high- and low-confidence groups, respectively; and repeatability of EPIC household activity: ρ = 0.81 (0.72 to 0.87) and 0.63 (0.48 to 0.74), respectively, and IPAQ vigorous activity: ρ = 0.58 (0.43 to 0.70) and 0.29 (0.07 to 0.49), respectively. Women were less likely than men to report high recall confidence of past-year activity (adjusted odds ratio = 0.38; 0.18 to 0.80). Conclusions: Confidence ratings could be useful as indicators of recall accuracy (ie, validity and repeatability) of physical activity measures, and possibly for detecting differential measurement error and identifying questionnaire items that require improvement.