Monitoring therapy for congenital adrenal hyperplasia (CAH) due to 21-hydroxylase is difficult, although plasma determinations of 17α-hydroxyprogesterone (170HP), Δ4-androstenedione (Δ4A) and testosterone are helpful. We have studied the usefulness of monitoring plasma 3α-androstanediol glucuronide (3α-AG) in a group of 24 CAH patients aged from birth to 18 years. For comparison, normal values for age and pubertal stage were determined in a control group of 115 girls and 118 boys. Mean plasma levels were higher during the first year of life, decreased to a nadir between 1 and 4 years, and increased steadily thereafter; there was also a significant increase with pubertal stage. In 24 pairs of blood samples obtained at the time of venopuncture and 2 h after, 3α-AG levels did not change (p > 0.05) demonstrating that 3α-AG levels were not affected by stress. In the patients with CAH, positive correlations between plasma 3α-AG and Δ4A (females, r = 0.73; males, r = 0.98), 170HP (females, r = 0.58; males, r = 0.84) and testosterone (females, r = 0.83; males, r = 0.97) were observed. Concordance between 3α-AG and Δ4A was observed in 90% of all samples, and in 91 % between 3α-AG and testosterone. Our study demonstrates that 3α-AG is a valid marker of control and its determination appears to be a reliable tool to monitor CAH.