Serum 3 alpha-androstanediol glucuronide (3 alpha-diol G) measurements may indicate the extent of androgen metabolism and action in target tissues. To test this supposition we measured serum 3 alpha-diol G concentrations in 23 women with congenital adrenal hyperplasia due to 21-hydroxylase deficiency, including 13 with the salt-losing and 10 with the simple virilizing form, while they were receiving glucocorticoid and, in some cases, mineralocorticoid therapy. Their mean age was 28.3 yr (range, 17.9-38.7 yr). Twenty-four-hour urinary 17-ketosteroid excretion, plasma androstenedione and testosterone levels, and serum 3 alpha-diol G levels were measured during the follicular phase. The values were within or below the normal range in 87%, 78%, 70%, and 91% of the patients, respectively. By contrast, plasma 17-hydroxyprogesterone levels were normal in only 12% of the patients. Serum 3 alpha-diol G levels correlated well with 24-h urinary 17-ketosteroid excretion (r = 0.75) and plasma 17-hydroxyprogesterone (r = 0.77), androstenedione (r = 0.84), and testosterone (r = 0.93) levels. The serum 3 alpha-diol G levels were not significantly different in the women with the salt-losing form and those with the simple virilizing form. However, they were significantly lower (P less than 0.05) in the women with normal menses compared to those with abnormal menses. This finding underscores the validity of serum 3 alpha-diol G measurements as indicators of androgen production and metabolism in women. The excellent correlation between the serum 3 alpha-diol G levels and standard measures of control indicates that the former measurement may be a useful adjunct in the management of women with congenital adrenal hyperplasia.