The greatest advances in our understanding of the role of estrogens in growth and skeletal maturity have come from male and female patients who are devoid of estrogen or have estrogen insensitivity. Among males, the absence of an estrogen effect results in lack of completion of skeletal maturity and continued growth into adult life, resulting in very tall stature. This has been exemplified in one male with an estrogen-receptor defect and one male with aromatase deficiency. In females with aromatase deficiency, androgen levels are excessive, causing inappropriate virilization but failing to induce skeletal maturity. Thus, estrogen is the hormone necessary for normal skeletal growth and maturity. Estradiol concentrations measured using an ultrasensitive assay are similar in males and females at the time of the pubertal growth spurt; these data are consistent with the premise that estrogen has a role in growth.