To evaluate the usefulness of blood testosterone (T) in monitoring the effects of therapy in congenital virilizing adrenal hyperplasia due to 21- or 11- hydroxylation defect (CVAH), T levels were measured on 45 occasions in 13 patients with CVAH; 32 urinary 17-ketosteroid levels and 31 preganetriol values were available for comparison. Bone age levels, growth data, and medication are listed to help assess the clinical state of the patient at the time of each T determination. Blood T values were above normal for age and sex in untreated patients with CVAH and declined with glucocorticoid suppression. A blood T value of 20 ng/100 ml appeared to distinguish between well-controlled cases and those with inadequate steroid suppression. Serial measurement of blood T in girls and in prepubertal boys with CVAH provides assistance in evaluating chemical control of the disease, particularly when accurate 24-h urine collections cannot be obtained for 17-ketosteroid and pregnanetriol assessments.