Objective: To examine the validity of self reported height and weight data reported over the telephone in the 1997 NSW Health Survey, and to determine its accuracy to monitor overweight and obesity in population surveys. Method: Self-reported and measured heights and weights were collected from 227 people living in Western Sydney, who had participated in the NSW Health Survey 1997. Results: Self-reported (SR) weights and heights led to misclassification of relative weight status. BMI, based on measured weights and heights, classified 62% of males and 47% of females as overweight or obese, compared with 39% and 32%, respectively, from self-report. Conclusions: Caution should be used when interpreting SR height and weight data from surveys, because BMI derived from these is likely to underestimate the true prevalence of overweight and obesity. Implications: SR data have a place in nutrition monitoring because they are relatively inexpensive and easy to collect. However, classifying people into weight categories on the basis of accepted cutpoints, using SR heights and weights, yields inaccurate prevalence estimates. Periodic sub-studies of the validity of SR heights and weights are needed to indicate the extent to which the validity of SR is changing.