The purpose of this study is to see if Arimidex, an aromatase inhibitor, can delay epiphyseal fusion and increase predicted adult height in boys who are growth hormone deficient, in puberty, and who are taking growth hormone. This is a double blind, placebo controlled 3 year trial.

Inclusion Criteria:

• Growth hormone deficient by formal testing with two provocative agents.
• Treated with growth hormone for a minimum of 6 months prior to study entry.
• Growth hormone doses must be maintained at 0.2-0.4mg/kg/wk while in protocol.
• Stable organic pathology
• Presence of puberty [genital Tanner Stage > II (>4cc testicular volume)]
• Bone age (BA) > or = 11.5 years and < 15 years

Exclusion Criteria:

• Participation in any other trial involving hormone therapy for at least 6 months prior.
• Chronic illnesses requiring long term medication that impair growth. (Stable patients with occasional asthma, patients on Ritalin or Adderall or patients on topical acne medication may be included).
• Hereditary disease diagnosed clinically.
• Moderate to severe scoliosis.