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