An initial high-dose treatment of glucocorticoid has been proposed to prevent chronic androgen excess, improving the final height prognosis of 21-hydroxylase deficiency (21-OHD) patients. In Japan, it is recommended to use an extremely high-dose of hydrocortisone (HDC) (100-200 mg/m(2)/day) for initial treatment by the Japanese Society for Pediatric Endocrinology. However, a precise evaluation of the treatment has not been carried out. In this study, we retrospectively analysed the effects of initial high-dose HDC therapy on the linear growth of classical 21-OHD patients discovered by newborn screening. Thirty patients (14 females) were eligible for this study, all of whom were initiated with high dose HDC therapy. The height standard deviation score (Ht-SDS) was 0.76 ± 0.65 at birth, and decreased to -1SD or less until the age of 12 months, subsequently catching up by 3 years of age (-0.56 ± 0.76). The growth pattern and the height at the age of two years were very similar to those previously observed in patients without initial high dose HDC therapy. We did not find any significant difference in growth retrospectively between the high- or low-dose HDC group (initial treatments of ≥150 mg/m(2)/day and 100 mg/m(2)/day, respectively). Bone ages did not exceed chronological ages at the ages of three and six years. Our data suggest that an initial high-dose HDC treatment does not profoundly affect linear growth during first three years of life and that the treatment could be a valuable option for 21-OHD patients without having an obvious adverse effect on linear growth.