Estimates of body size are often expressed as a ratio of actual size [body perception index or BPI = (estimated size/actual size) x 100%]. In this article, we examine the possibility that overestimation of body size in patients with anorexia nervosa, as measured by the BPI, is due to their smaller body size rather than to their being anorexic. Using 50 mean body sizes derived from seven studies we investigated whether the error of estimation is a constant proportion of the body size to be estimated, as the use of the BPI assumes. A negative linear relation between BPI and actual body size was found, confirming that smaller size is associated with greater overestimation. However, although both groups showed a strong tendency to overestimate smaller sizes, anorexic subjects showed even greater overestimation than controls. Hence, overestimation of body size in AN can only partially be accounted for by the smaller body size of anorexic patients. Recommendations for future use of the BPI are put forward.