This research was conducted to investigate the relationship between plasma hormone level during activation of hypothalamic-pituitary-testicular axis at the postnatal period and at puberty in unilateral cryptorchidism. Plasma testosterone and estradiol levels of 80 patients with unilateral cryptorchidism at different ages (range: 6 months-12 years) were measured. The mean plasma testosterone level is 40 (15-60) pg/ml at 6 months of age, 55 (30-120) at ages between 9 and 12 years, and 20 (11-22) at ages between 1 and 9 years. The mean plasma estradiol level is 12, 11 and 11 (5-24) pg/ml, respectively, in these groups. The patients with unilateral cryptorchidism do not have similar peaks of plasma estradiol level as plasma testosterone level at the postnatal period and at puberty. Peak of plasma testosterone at puberty occurs if the patients were not operated on.