Technical Abstract: Little information about relationships between the physical activity and sedentary behaviors of young children is available in the literature. We therefore examined how sedentary behaviors, TV watching, and encouragements and discouragements for activity were associated with physical activity (as measured by observation and heart rate monitoring) among a tri-ethnic cohort of 149 three- to four-year-old children, that we followed for 3 years. The results showed that sedentary behavior predicted observed activity in Years 2 and 3 (r = –.672 and –.831, respectively, R2 = .577 and .775, respectively). Similar results were obtained for heart rate monitoring. Reducing the time children spend in sedentary behavior might result in increased physical activity.