Objective 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. Design/patients/measurements 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. Results 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. Conclusions 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.