Aim: Children treated with a growth hormone (GH) for idiopathic growth hormone deficiency (IGHD) may be monitored with the first-year prediction model from the Pfizer International Growth Database (KIGS) using auxology, age, GH dose and the maximum GH concentration from a stimulation test (GH(max)stim). We tested the hypothesis that using a 12-hour spontaneous profile (GH(max)12h) would be as accurate. Methods: We studied 98 prepubertal Swedish children (78boys) aged2-12 years enrolled in KIGS. The first-year growth was predicted using the GH(max) from the GHprofile and a stimulation test, and both of these were compared separately with the observed growth response. Results: The increased height observed in the first year was 0.74 standard deviation scores (SDS), and the studentised residuals for the predicted and observed growth with GH(max)stim (-0.16 SDS) and GH(max)12h (-0.22) were similar. Individual predictions calculated with stimulated or spontaneous GH(max) showed a significant correlation (r = 0.80). Conclusion: We validated the KIGS IGHD prediction model and found that the stimulated GH(max) peak can be reliably replaced by the GH(max) 12h with similar accuracy. This makes the model more accessible for clinicians, who can then provide realistic expectations for the growth response during the first year of treatment.