The effect of recombinant human growth hormone on children with Down syndrome who had growth retardation and microcephaly was examined. Thirteen children with trisomy 21 without congenital heart disease who were short for age (-1.19 to -3.5 standard deviation score) and microcephalic (-1.58 to -6.60 standard deviation score) were given recombinant human growth hormone, 0.1 mg/kg subcutaneously, 3 days a week for 1 year. Before treatment, peak serum growth hormone concentrations were less than 10 micrograms/L after levodopa and clonidine stimulation tests in five patients, after clonidine in three patients, and after levodopa in three patients. Three patients had nocturnal integrated growth hormone concentrations of 0.5, 1.5 and 0.65 micrograms/L, respectively. The mean growth rate before treatment was 5.4 +/- 1.6 cm/yr and increased to 12.2 +/- 3.2 cm/yr (p less than 0.001) after 12 months of recombinant human growth hormone treatment. The mean head circumference standard deviation score before treatment was -3.1 +/- 1.3 and increased to -2.3 +/- 1.2 (p less than 0.001) at 12 months. Bone age before and 1 year after treatment increased in correspondence with chronologic age. Plasma hemoglobin A1c concentration was normal during treatment with recombinant human growth hormone. The mean plasma concentrations of insulin-like growth factor I at baseline and at 12 months were 0.54 +/- 0.19 U/ml and 1.25 +/- 0.97 U/ml, respectively (p less than 0.02). We conclude that recombinant human growth hormone therapy can result in a significant increase in annual growth rate and head circumference in children with Down syndrome, without significant side effects.