A telephone survey of 20 English intensive care units (ICUs) confirmed that visual estimation of patient weight is often performed. Four experienced intensive care staff (three doctors and one nurse) estimated the weight and measured the height of 30 volunteers and the estimates were compared with accurate reference measurements. The estimates were shown to be significantly inaccurate for individual observers. We consider the degree of inaccuracy to be of clinical importance. However, pooling the individual estimates of weight as mean values improved accuracy. Recommendations are made concerning the performance of height and weight determination.