To know the effect of the age factor on growth response to growth hormone (GH) therapy in patients less than three years of age with complete idiopathic growth hormone deficiency (GHD). One hundred thirty-seven prepubertal children with complete idiopathic GHD from the database of the International Growth Cooperative Study (ICGS) of Japan for more than two years were analyzed. The patients were divided into four groups according to age at the start of GH therapy; group 1, < or = three years, group 2, three to < or = six years, group 3, six to < or = nine years, group 4, >nine years. In group 1 the mean birth weight SDS of -0.17 was not the lowest. Patients in group 1 had the lowest HSDS (-4.53) at the start of the therapy (P<0.01). During two years of the treatment, the mean HSDS improved from -4.53 to -2.15 with an increase in mean SD score of 2.12 in group 1, but in group 4 it was only 0.92 (P<0.01). In group 1, the mean HSDS declined from -0.17 at birth to -4.53 progressively. Delta height SDS during the first year of treatment in Group 1 was 1.59 SD, which is the highest. The decrease in %overweight during the first year showed that in group 1, the reduction in adiposity was the greatest among the groups (P<0.05). Our data showed that in children with complete GHD less than three years of age, there was severe growth failure, and the onset was not in the prenatal period, but in the postnatal period. Early detection of GHD and therapy with GH is important because normalization of height may be achieved earlier than in older age groups.