Childhood obesity is common in New Zealand with one in three 4-5 year-old children identified as overweight or obese in the before school check (B4SC) programme. Recently, the use of BMI for assessing childhood obesity in the B4SC programme has been questioned. This article provides evidence in support of the assessment of BMI during the B4SC, including specific key points: 1. BMI is currently the only appropriate field tool for assessing overweight and obesity.2. Our New Zealand data show that BMI is reliable at measuring adiposity in all ethnic groups. 3. High childhood BMI often leads to adult obesity and is associated with increased adult morbidity and mortality. 4. We believe parents do want to know information regarding their child's obesity risk, based on recent findings in our New Zealand study.