As there are no standard criteria for monitoring suppression during treatment of central precocious puberty (CPP) with gonadotrophin-releasing hormone analogues (GnRHa), we assessed the use of pelvic ultrasound examination for this purpose. In 31 girls with CPP, transabdominal pelvic ultrasound examination was performed before initiation of therapy with GnRHa, after approximately 3 and 6 months, at last treatment visit and after its discontinuation. Three months after treatment initiation, there was a significant decrease in most uterine and ovarian parameters, with at least three parameters decreased in each patient. Endometrial echo was found in 80% of girls before therapy, in 52% (13/25) after 3 months of therapy, in 24% (6/25) after 6 months and in none on the last treatment visit (P < 0·001). In the course of treatment (mean therapy duration 2·5 ± 0·9 years), uterine parameters showed no significant change; ovarian parameters initially showed a decrease and later a modest increase although they were still smaller than before therapy. Within 3-11 months after therapy discontinuation, endometrial echo was detected in 85%, with a significant increase in uterine and ovarian parameters. This prospective study indicates that comparison of individual ultrasound measurements throughout the course of treatment in girls with CPP provides a valid assessment of the suppression of the hypothalamo-pituitary-gonadal axis achieved GnRH therapy. Uterine parameters and absence of endometrial echo were found to be better indicators of adequate suppression than ovarian parameters.