Small stature is associated with low growth hormone secretion, but in most cases the reason is unknown. The commonest cause of hormone insufficiency is autoimmunity. and autoantibodies to hormones are often found where there is autoimmune disease of the corresponding gland.Displaceable growth hormone binding by the sera of 125 short (<3rd centile) but otherwise normal school entrants was significantly higher (P<0.05) than by the sera of 100 age-matched children of normal height (10th-90th centile), and binding in 21 (17%) of the small children exceeded the upper limit of 95% of the normal population. Furthermore, urinary growth hormone excretion was significantly lower in the small children (total overnight output 0.6–1.7 ng) compared with controls (1.5–3.7 ng) (P<0.05) even when corrected for body surface area. Thus, growth hormone binding and growth hormone excretion discriminated between two groups of children selected only on the basis of height. The assay used for growth hormone binding was specific for IgG. suggesting that the binding factor was antibody. Autoimmunity merits further investigation as a basis for poor growth.