Background: The purpose of this study was to examine the temporal patterns of preschoolers' physical activity (PA) levels during a typical outdoor free playtime. Methods: Baseline playtime accelerometer counts (4.3 +/- 0.8 days) from 3 preschool PA intervention studies were used (n = 326 children, age = 4.0 +/- 0.8 years). Data were collected using 15-second epochs and classified into sedentary, light, or moderate-to-vigorous physical activity (MVPA). Patterns of change during playtime were analyzed using orthogonal polynomial comparisons. Results: For all ages, there was a U-shaped pattern of change for the percent of epochs classified as sedentary [F(1, 323) = 47.12, P .001) and an inverted U-shaped pattern of change for the percent of epochs classified as MVPA [F(1,323) = 32.15, P .001]. Age-stratified analyses indicated that the 3-year-olds maintained the decrease in sedentary time [F(2,323) = 6.408, P = .002] and the increase in MVPA [F(2,323) = 3.2, P = .04] to a greater extent than the 4- and 5-year-olds. Conclusions: Preschool children gradually became more active during the first 10 to 15 minutes of outdoor gross motor playtime and less active over the final 10 to 15 minutes of playtime. During the second half of playtime 3-year-olds maintained these changes to a greater degree than 4- and 5-year-olds. (author abstract)