Objective: To evaluate whether genotype differences can explain the clinical variability of non-classical steroid 21-hydroxylase de®ciency (NC21-OHD) and to determine if genotype is related to ethnic origin. Design: Genotyping for mutations in the steroid 21-hydroxylase (CYP21) gene was performed in 45 unrelated Israeli Jewish patients (nine males) with NC21-OHD (60 min 17-hydroxyprogesterone (17-OHP), 45±386 nmol/l) who were referred for evaluation of postnatal virilization or true precocious/ early puberty. Eleven siblings diagnosed through family screening were genotyped as well. Methods: Patients were divided by genotype into three groups: (A) homozygous or compound heterozygous for the mild mutations (V281L or P30L) (n=29; eight males); (B) compound heterozygous for one mild and one severe mutation (Q318X, I2 splice, I172N) (n=12; no males); (C) mild mutation detected on one allele only (n=4; one male; peak 17-OHP 58±151 nmol/l). We then related the genotype to the ethnic origin, clinical phenotype and hormone level. Since group C was very small, comparisons were made between groups A and B only. Results: At diagnosis, group B tended to be younger (5.863.0 vs 8.164.3 years, P 0.09), had greater height SDS adjusted for mid-parental height SDS (1.661.1 vs 0.761.4, P 0.034), tended to have more advanced bone age SDS (2.961.5 vs 1.762.1, P 0.10) and had a higher peak 17-OHP level in response to ACTH stimulation (226692 vs 126662 nmol/l, P < 0.01). Group B also had pubarche and gonadarche at an earlier age (5.162.4 vs 7.462.2 years, P < 0.01 and 7.461.8 vs 9.961.4 years, P < 0.001, respectively) and a higher rate of precocious puberty (50 vs 17%, P 0.04). Stepwise logistic regression analysis (excluding males) yielded age at gonadarche as the most signi®cant variable differentiating the two groups, with a positive predictive value of 86% for a cut-off of 7.5 years. Conclusions: The ®ndings suggest that genotype might explain some of the variability in the phenotypic expression of NC21-OHD. Compound heterozygotes for one mild and one severe mutation have a higher peak 17-OHP associated with pubarche and gonadarche at an earlier age and more frequent precocious puberty. Hence, the severity of the enzymatic defect might determine the timing and pattern of puberty.