Background  Women with 21‐hydroxylase deficiency present much variability in external genitalia virilization, even among those with similar impairments of 21‐hydroxylase (21OH) activity.

Objective  To evaluate if the number of CAG (nCAG) repeats of the androgen receptor gene influences the degree of external genitalia virilization in women with CYP21A2 mutations, grouped according to impairment of 21OH activity.

Patients  The nCAG was determined in 106 congenital adrenal hyperplasia (CAH) patients and in 302 controls. The patients were divided, according to their CYP21A2 genotypes, into Groups A and B, which confer total and severe impairment of 21OH activity, respectively.

Methods  The inactivation pattern of the X‐chromosome was studied through genomic DNA digestion with Hpa II. The CAG repeat region was amplified by polymerase chain reaction (PCR) and analysed by GeneScan.

Results  The nCAG and the frequency of severe skewed X‐inactivation did not differ between normal women and patients. The nCAG median in genotype A was 20·7 (IQR 2·3) for Prader I + II, 22·5 (3·6) for Prader III and 21 (2·9) for Prader IV + V (P < 0·05 for Prader III and Prader IV + V). The nCAG median in genotype B was 21·3 (1·1) for Prader I + II, 20·5 (2·9) for Prader III and 22 (2·8) for Prader IV + V (P > 0·05). A significant difference was found regarding the nCAG median in patients presenting Prader III from genotypes A and B.

Conclusions  We observed great variability in the degree of external genitalia virilization in both CYP21A2 genotypes, and we showed that the CAG repeats of the androgen receptor gene influences this phenotypic variability.