OBJECTIVES: To describe adherence with growth hormone (GH) treatment by age group among newly initiated patients. METHODS: Somatropin pharmacy claims were selected from July 1999 through May 2008 in PharMetrics, a national managed care dataset. Continuous eligibility for 6 months before and 12 months after the first GH prescription (i.e., index date) was required.  Patients were excluded if they had: claims for brands of somatropin used for HIV-associated wasting; potential data entry errors in key variables; or had a claim for GH product no longer on the market. Adherence was assessed using medication possession ratios (MPRs) defined as the sum of days supply divided by 360 days of observation and capped at 100%.  Evaluations by age group included average MPR, percentage of patients with at least 80% MPR, and percentage of patients who stay on therapy (i.e., persistence) for at least 80% of the observation period. RESULTS: A total of 1355 patients receiving GH met study criteria; 64.0% males and 36.0% females. The percentage of patients receiving GH by age category was as follows: <4 (5.2%), 4 to <13 (40.9%), 13 to <18 (34.8%), 18+ (19.1%). Average MPR was greatest in the <4 age group (87.2; SE 2.8), followed by 4 to <13 (82.9; SE 1.0), 13 to <18 (80.5; SE 1.1) and 18+ (68.1; SE 1.5).  Patients in the 18+ groups had significantly lower mean MPR (68.1% SD 27.2) than all other age groups (all above 80%; p<0.0001).  The percentage of patients with an MPR of at least 80% was 71.4%, 69.3%, 66.1% and 44.8%, respectively. Approximately 89% of the 4 to <13 age group remained on therapy (i.e., persisted) for more than 80% of the observation period versus only 64.5% of the 18+ group. CONCLUSIONS: Adherence with growth hormone decreases as age increases.  There is opportunity, however, for improvement in all age groups.