Three patients with XO sex chromosome constitutions had human growth hormone (HGH) measurements. Exogenous HGH was given to one patient to observe its effect on linear growth. All patients had normal HGH response to hypoglycemia or arginine maleate stimulation. The growth rate of the treated patient was followed during three one-year periods. During the control period the patient's growth rate was 0.23 cm/month. It increased to 0.5 cm/month while she was receiving HGH, 10 mg/ week. During the last period, she took small doses of estrogenic substances conjugated and grew 0.3 cm/ month. Although the patient with Turner's syndrome is genetically destined to be short, growth may be stimulated in some patients with exogenous HGH.