This study investigates parents' ability to correctly classify their child's weight status. The influence of parent and child socio-demographic and lifestyle factors on parental misclassification of their child's weight status is explored. A representative sample of Irish children (aged 5-12 (n = 596) years, aged 13-17 years (n = 441)) and their parents (n = 1885) were recruited to participate in a national dietary survey. Parental perceptions of their child's weight and their own weight were measured. Anthropometric measurements (weight and height) were objectively measured for parents and children. Body Mass Index (BMI) scores were derived and categorised as normal, overweight or obese using standard references. Over 80% of parents of overweight boys and 79.3% of parents of overweight girls reported their child's weight was fine for his/her height and age. Furthermore, 44.4% of parents of obese boys and 45.3% of parents of obese girls felt their child's weight was fine for their height and age. Parents were significantly less likely to be correct about their sons' weight status and more likely to be correct the older the child. Parents were over 86% less likely to be correct about their child's weight if their child was overweight and approximately 59% less likely to be correct if the child was obese, compared to parents of normal weight children. This research suggests that parents are failing to recognise overweight and obesity in their children with factors such as parental weight status, child's age and gender influencing this.