Spontaneous pubertal growth was studied in eight patients with the syndrome of androgen insensitivity to obtain information on the growth-promoting action of estrogens. In one additional patient (who had a gonadectomy before puberty), the effect of exogenous estrogens was studied. Mean age at peak height velocity (12.7 years) was closer to that in normal girls than to that in normal boys. Mean peak height velocity (7.4 cm/yr) was as in normal girls (7.3 cm/yr), but was lower than in normal boys (9.3 cm/yr). Bone age corresponded better to male standards. Mean adult height (172.3 cm) was lower than in normal men (-0.6 SD), but higher than in normal women (+1.4 SD). In the patient who had a gonadectomy, estrogen replacement caused a higher peak height velocity (12 cm/yr), but lower adult height (160.5 cm) than in the patients with intact gonads who received no treatments. We conclude that in normal girls, the pubertal growth spurt also results from the action of estrogens rather than of adrenal androgens. To ensure normal pubertal growth, physiologic estrogen replacement in hypogonadal females should be started at a bone age of about 11 years, and should not be delayed in the hope of achieving a greater mature height.