Theoretic concerns exist that children with chondrodysplasia will not grow in response to growth hormone (GH) therapy because of an inability of the abnormal growth cartilage to respond. Experience to date, however, suggests that there is an increase in growth velocity, especially during the first year of treatment, which may be beneficial. Growth has increased during the early phases of GH therapy in both patients with achondroplasia and patients with hypochondroplasia. Fourteen patients with achondroplasia in the National Cooperative Growth Study have been treated with an average dose of GH of 0.317 mg/kg per week for an average of 2.6 years and have gained an average of 0.7 SD in height. Twenty patients with hypochondroplasia in the National Cooperative Growth Study have been treated with an average dose of GH of 0.317 mg/kg per week for an average of 2.6 years and have gained an average of 0.7 SD in height. These data suggest that the abnormal growth cartilage in patients with chondrodysplasia responds to GH therapy. The effect on final height cannot be predicted with the currently available data.