Introduction:Thinness affects more children and adolescents than obesity. Thus, the aim of the study is to examine the recent estimates of thinness and associated risk factors, and to identify trends in thinness, among Greek schoolchildren. Methods:Epidemiological study. Population data from the recent estimates are derived from a school-based health survey polled in 2015 on 336,014 participants aged 4- to 17-years-old. To assess trends of thinness (1996-2015) we included a total of 300,104 children aged 8- to 9-years-old. Physical activity, dietary habits and sedentary activities were assessed through self-completed questionnaires. The gender and age-specific body mass index cut-off points proposed by International Obesity Task Force were used in order to define weight groups. Results:Percent 8.4% of girls and 6.5% of boys were thin (all grades included). The prevalence of thinness decreased with age more in boys (from 13.8% at 4-years-old to 5.1% at 17-years-old, p < 0.001), than in girls (from 10.9% at 4-years-old to 8.7% at 17-years-old, p < 0.001). Sufficient dietary habits (OR: 0.87, 95% CI: 0.77-0.97) and adequate physical activity levels (OR: 0.92, 95% CI: 0.85-0.99) were associated with decreased risk of thinness. Thin schoolchildren performed better in aerobic fitness test than normalweight ones. Between 1996 and 2015, thinness rates decreased from 8.0% to 6.5% in boys (p = 0.046) and from 10.6% to 8.4% in girls (p = 0.036). Conclusions:Our results suggest that thinness is a significant overlooked phenomenon. Although the prevalence of thinness has decreased the last two decades among Greek schoolchildren, actions need to be taken from public policy makers in order to establish and maintain a healthy body weight.