Background: The ToyBox-study developed an evidence- and theory-based intervention to improve preschoolers' energy balance-related behaviours - including physical activity (PA) - by targeting the kindergarten environment and involving their parents/caregivers. The present study aimed to examine the effect of the ToyBox-intervention on increasing Belgian preschoolers' objectively measured PA levels. Methods: A sample of 472 preschoolers (4.43 +/- 0.55 years; 55.1% boys) from 27 kindergartens (15 intervention, 12 control kindergartens) in Flanders, Belgium were included in the data analyses. Preschoolers wore an ActiGraph accelerometer for six consecutive days and were included in the data analyses if they had a minimum of two weekdays and one weekend day, both at baseline and follow-up (one year later). Preschoolers' PA outcomes were estimated for an average day, weekday, weekend day, during school hours, and during after school hours. To assess intervention effects, multilevel repeated measures analyses were conducted for the total sample, and for sub-groups (according to sex, kindergarten levels of socio-economic status (SES) and risk groups (low levels of PA at baseline)) of preschoolers. Results: Small intervention effects were found in the total sample. Most intervention effects were found in boys and in preschoolers from high SES kindergartens. Boys from the intervention group had an increase in vigorous PA (ss = 1.47, p = 0.03) and moderate-to-vigorous PA (ss = 1.27, p = 0.03) from baseline to follow-up, whereas PA levels in boys from the control group stagnated or decreased. In preschoolers from high SES kindergartens, the largest effects were found for PA outcomes during school hours and during after school hours. Conclusion: The results from the Belgian sample demonstrate that effects of the PA-component of the ToyBox-intervention on objectively measured PA were found in preschool boys and in preschoolers from high SES kindergartens, which means that the ToyBox-intervention was mainly effective in those ...