In diagnosing patients with short stature, it is very important to identify patients with disproportionate short stature since these patients are likely to have genetic and/or metabolic bone disorders rather than endocrinological short stature, and the prognosis and the treatment are quite different. Traditionally, to identify patients with disproportionate short stature, arm span or pubis-to-floor length are compared with the total height (1, 2) or sitting heights are compared with standards (1). Although there are no standardized statistics of arm span in Japanese children, arm span is believed to be close to the height of an individual both from the studies on white children (3, 4) or on multiracial populations (5). Usually the diagnosis of disproportionate short stature is easily made when the patient has abnormally short limbs compared with height. However, there is a diagnostic challenge in young infants since in this age group, the body proportions are different from those of older children and adults. The proportion of the head is larger in infants giving the impression of shorter limbs compared with those of older children. As an aid in the diagnosis of infants with disproportionate short stature, we attempted to establish the standards of arm span in Japanese infants. We selected normal Japanese infants at 1 month and 4 months of age, since these are the months when most Japanese infants undergo routine well baby visits under the current health maintenance policy of the Japanese government.