The objective of this study was to investigate the effect of administration of recombinant human growth hormone (hGH) in patients with Noonan syndrome. hGH was administered (0.5 IU/kg/week) to 15 patients with Noonan syndrome over a 2 year period. Average patient age prior to therapy was 7.5 +/- 2.5 (mean +/- SD) yr, the height SD score was -2.8 +/- 0.7, and the pretreatment height velocity and bone age were 4.8 +/- 1.0 cm/yr and 5.8 +/- 2.1 yr, respectively. The height velocity in the year prior to treatment, and 0-12 and 12-24 months after commencing treatment was 4.8 +/- 1.0 cm/yr, 7.0 +/- 1.2 cm/yr, and 5.5 +/- 0.6 cm/yr, respectively. The height velocity in the first year of treatment was significantly greater (P = 0.0001, n = 14) than the pretreatment value, but there was no significant difference in the second year. The height SD scores at the commencement of treatment, and after 12 and 24 months of treatment were -2.8 +/- 0.7, -2.4 +/- 0.7, and -2.2 +/- 0.5, respectively. Bone age advanced by 1.1 +/- 0.5 yr in the 12 months after commencing treatment. We conclude that the use of hGH may be beneficial in the treatment of Noonan syndrome, although further research is required.