This study compares levels of physical activity of 9201314-year-old children from a self-reported time diary with those measured using an accelerometer. Children (N = 92) wore an accelerometer for one weekend day and completed a 24-hour time diary for that day. The time children spent in moderate to vigorous physical activity from time diaries was moderately highly correlated with the measured results. Self-reported and objective measures of intensity were correlated, but the correlation varied substantially by activity and characteristics of the family. This study provides empirical evidence to support the validity of time-diary estimates when accelerometer data are not available.