OBJECTIVE: Information about the accuracy of family informant estimates of height and weight should assist investigators in evaluating the costs and benefits of using this type of data in genetic study designs. DESIGN AND METHOD: To assess the accuracy of family informant estimates, 374 first-degree relatives from 94 Caucasian families, gave estimates about the heights and weights of their first degree relatives. These estimates were compared with measured heights and weights to determine their accuracy. RESULTS: Informant estimates were highly predictive of measured heights (r=0.95), and weights (r=0.94), but informants systematically overestimated heights (mean=1.4 cm) and underestimated weights of their family members (mean=4.1 kg). CONCLUSIONS: On average, height estimates were generally within 1% of the measured height and weight estimates were within 3-5% of the measured weight. Therefore, these proxy measures can provide useful data, when measured or self-reported heights and weights are not available.