Abstract Background Childhood overweight has a negative impact on children's (mental) health. Although the prevalence of overweight among children is high in Western countries, the increasing trend seems to be levelling off since 2000. The purpose of this study is to examine this trend over the past two decades in Belgium and whether this trend differs by the parents’ socioeconomic status (SES). Methods Data from the Belgian Health Interview Survey (HIS), a representative cross-sectional household survey conducted in 1997, 2001, 2004, 2008, 2013 and 2018, was used. Study participants include children aged 2-17 years. The highest educational level among parents is used as a proxy for SES and is dichotomized into lower (at most a higher secondary degree versus high (higher education). Since the BMI (kg/m2) in children is instable, age/sex-specific cutoff points are used to estimate the weighted prevalence (95% confidence interval) of self-reported overweight (including obesity), by SES and year. Significant differences (p<.05) of the association between overweight and SES 1997 is assessed through logistic regression controlled for age and sex using 1997 as reference year. Results The overall prevalence of overweight among children is significantly higher in 2018 (19.0% (16.4-21.6%)) than in 1997 (13.6% (11.2-16.0%)), but no difference compared to the intervening years. Among the lower educated, this prevalence increased from 16.8% (13.2-20.4%) in 1997 to 27.8% (23.3-32.4%) in 2018; and among the high educated from 8.8% (6.0-11.5%) to 12.9% (10.0-15.8%). Compared to 1997, the difference between the low and the high educated increased significantly since 2008. Conclusions Two decades of HIS have shown that the SES is becoming an even more important risk factor for overweight among children. Health promotion on nutrition and physical activity at school has the potential to reach children from all socioeconomic backgrounds, making it an important means of tackling health inequalities. Key messages • Efforts to ...