Decreased physical activity (PA) is likely a contributor to the rising prevalence of obesity in children. Lack of valid and acceptable measures of PA has been an issue in studies involving young children. The Actical accelerometer is a promising tool for measurement of PA in young children. The purpose of this study was to calibrate and validate the Actical accelerometer for use with 3- to 5-yr-old children. Eighteen preschool children wore an Actical accelerometer and a Cosmed portable metabolic system during a period of rest, while performing three structured activities in a laboratory setting (used for calibration), and during 20 min each of unstructured indoor and outdoor activities at their preschool (used for cross-validation). Expired respiratory gases were collected, and oxygen consumption was measured on a breath-by-breath basis. Accelerometer data were collected in 15-s intervals. For the accelerometer calibration, the correlation between VO2 and counts was r = 0.89 across all activities. The calibration equation established was VO2 = counts x 15 s(-1) (0.01437) + 9.73 (R2 = 0.96, SEE = 3.02). The cut-point for moderate activity (20 mL x kg(-1) x min(-1)) was 715 counts x 15 s(-1) (sensitivity 97.2%, specificity 91.7%), and the cut-point for vigorous activity (30 mL x kg(-1) x min(-1)) was 1411 counts x 15 s(-1) (sensitivity 98.2%, specificity 61.1%). For the cross-validation, the intraclass correlation coefficient was R = 0.59 and the Spearman correlation coefficient was R = 0.80 (P < 0.001) between measured and predicted VO2. Percentage of agreement, kappa, and modified kappa for moderate activity were 0.73, 0.40, and 0.46, respectively. For vigorous activity, the same measures were 0.85, 0.26, and 0.71, respectively. The Actical accelerometer is a valid tool for measuring PA in young children.