Introduction. Many parents have a misperception of their children's body size; in general, they underestimate overweight and obesity. Objective. To identify the difference between parents' perception of their children's nutritional status and measured body mass index. Population and methods. An analytical, descriptive and cross-sectional study was conducted among parents of children aged 2 to 6 years old. Children's body mass index was measured, and parents' perception was assessed using a visual scale of body size pictograms (drawings of body figures equivalent to body mass index percentiles). Results. A total of 605 children and their parents were assessed. Seventy-four (12.2%) were overweight and 87 were obese (14.3%). There were 161 overweight or obese children, but 98.8% (159) of parents underestimated their children's nutritional status. Parents' underperception of their children's body size accounted for an OR= 2.1 ± 0.5, p = 0.002 (95% confidence interval [CI]: 1.32-3.32) for obesity and an OR = 4.42 ± 1.2, p < 0.001 (95% CI: 2.631-7.439) for overweight. Conclusions. Among parents of overweight and obese children, 98.8% (159) underestimated their children's weight status. Such underestimation by parents may be a significant risk factor for the development and/or persistence of overweight and obesity in their children.