We evaluated the response to growth hormone (GH) therapy in 150 children (97 boys) with Noonan syndrome (NS) by analyzing growth data from children with NS who were enrolled in the National Cooperative Growth Study and compared those data with National Cooperative Growth Study growth data from children with idiopathic growth hormone deficiency (IGHD) and Turner syndrome (TS). Children with NS were significantly shorter than those with IGHD and TS. The annualized growth rates for years 1, 2, 3, and 4 of therapy in patients with NS who were naive to previous GH therapy were significantly greater than baseline. Their growth rates for years 1, 2, 3, and 4 were intermediate between those in children with IGHD and TS and were significantly different from both. A significant improvement occurred in height SD scores for those 42 children with NS who have been monitored for at least 4 years of GH therapy. Three of six boys with NS for whom adult height data were available exceeded their pretreatment predicted heights.