Inclusion Criteria:

1. Signed informed consent
2. Children aged ≤3yrs old and not above 10 yrs for girls or 11 yrs for boys
3. Idiopathic growth hormone deficiency confirmed during the pre-screening period by a standard GH stimulation test (defined as peak level of <10ng/ml pr lower if so required by the country specific board(s)
4. Height SDS <-2 SD of ref value for CA
5. Height velocity SDSCA ≤ 0 SD of ref value for at lease 6 months prior to pre-screening
6. Height recorded for at least 6 months but not more than 18 months of pre-screening
7. The difference between CA-BA≥ 1
8. A positive locally performed GH stimulation test (defined as a peak plasma level of<9ng/ml or lower if so required by the country specific board(s)) prior to the pre-screening

Exclusion Criteria:

1. BA above 9 yrs for girls and 10 yrs for boys
2. Puberty Tanner stage >1
3. Weight <12 Kg at screening
4. Any prior treatment with GH
5. Closed epiphysis
6. Any diagnosed or suspected syndrome (e.g. Silver -Russell, Turner's or seckel syndrome) which possibly could affect growth
7. Any other diagnosed or suspected endocrine or metabolic disorder
8. Any diagnosed or suspected sever chronic disease
9. Clinical signs of dysmorphic features, malformations or mental retardations
10. Growth failure due to other disorders
11. Previous or present use of drugs that could interfere with GH treatment (e.g. steroids)
12. Diagnosed malignant disease
13. Any abnormal CS lab results that requires further investigation
14. Receipt of an investigational drug within the last 28 days preceding screening or longer if considered possible to influence the outcome of the current trial
15. Any knowledge of hypersensitivity to somatropin or any of the excipients of Zomacton or Genotropin