Introduction: It is well established that regular physical activity (PA) contributes to lower levels of morbidity and mortality. However, little is known about the stability of very young children's PA habits across seasons and years. The aims of this study were to 1) examine the influence of season and increasing age on objectively assessed PA in preschool children and 2) examine the stability of young children's PA rankings during 1 yr. Methods: The PA levels of preschool (3-and 4-yr-old) children were measured, using 6-d pedometer step counts, during winter and spring (n = 85, 52 boys). PA levels were measured again 1 yr after the spring data collection when the children had entered primary school (n = 37, 22 boys). Parents completed questionnaires to assess attitudes toward PA, PA habits, and demographic information in the winter of the first year and the spring of the second year. Results: Young children take approximately 2000 (20%) fewer steps per day in winter than in spring with a rank order stability between the two measures of r = 0.04 (P < 0.01). A modest degree of the observed intrachild or seasonal variation was related to the amount of time fathers played with their children (P < 0.05) and the availability of a safe place for children to play (P < 0.05). Children took approximately 2300 (20%) more steps per day at age 5 compared with age 4 (P < 0.01). The rank order stability of young children's PA during this period was low with correlations ranging from 0.01 to 0.15. Conclusions: Results suggest that a one-off assessment of PA is unlikely to be representative of a young child's activity during 1 yr and that PA tracks poorly from age 4 to 5. © 2012 by the American College of Sports Medicine.