It remains controversial if glucocorticoid replacement therapy impairs bone mineral density (BMD) in young patients with 21-hydroxylase deficiency. We aimed to analyze the impact of treatment variables, phenotype and genotype on BMD and bone metabolism in these patients. Cross-sectional study. Twenty-eight Caucasian patients with classical 21-hydroxylase deficiency (5-39 years). Clinical parameters, hormonal status, osteocalcin (OC), C-terminal telopeptide of type I collagen (CTX), genotype and lumbar BMD (Z-scores) were assessed. Cumulative and mean hydrocortisone equivalent doses were calculated for the entire treatment period. Patients with severely reduced BMD Z-scores (< or = -2.5 SD) had significantly higher mean/cumulative glucocorticoid doses compared to patients with moderately reduced (P = 0.003/P = 0.026) and normal Z-scores (> -1 SD) (P = 0.005/P = 0.011). Mean hydrocortisone equivalent doses > 20 mg/m(2)/day led to significantly lower lumbar BMD Z-scores (-2.16 +/- 1.4 SD) vs. doses </= 20 mg/m(2)/day (-0.59 +/- 1.25 SD) (P = 0.008). BMD correlated negatively with mean/cumulative glucocorticoid doses and treatment duration. OC (86.45 +/- 37.45 ng/ml) and CTX (1.45 +/- 0.43 ng/ml) were significantly increased compared to an age- and sex-matched control group in patients with active growth; only CTX was slightly increased in patients who completed growth. High cumulative and mean glucocorticoid doses negatively impact on BMD in children and young adults with classical 21-hydroxylase deficiency. Substitution therapy should be adapted particularly at this life period to prevent bone loss.