Pelvic ultrasonography was systematically performed on 33 girls with idiopathic central precocious puberty to investigate the impact of treatment with gonadotropin-releasing hormone analogues on female internal genitalia. All girls were treated with a long-acting gonadotropin-releasing hormone analogue (Decapeptyl Depot; Ferring Co., Copenhagen, Denmark) 75 micrograms/kg every 4 weeks. Before, during, and after treatment, pelvic ultrasonography was performed and ovarian and uterine volumes were calculated. The size of follicles > 5 mm were accurately measured. The results were related to a normative study of healthy Danish schoolgirls. Our data demonstrated that ovaries and uterus are enlarged in a significant number of girls (50%) with the diagnosis of central precocious puberty at the time of diagnosis. Median ovarian volume at time of diagnosis was 1.1 standard deviation scores (range -0.6 to 3.2 SD), median uterine volume was 1.8 standard deviation scores (range 0.0 to 3.5 SD). Within 3 months of treatment, both ovarian and uterine volumes decreased significantly (p < 0.01) to normal values appropriate for age. Median ovarian volume after 3 months of treatment was 0.0 SD (range -2.4 to 1.5 SD); median uterine volume was 0.7 SD (range -0.6 to 4.1 SD). Ovarian and uterine volume remained within normal range (< 2 standard deviation scores) after discontinuation of treatment. Follicles and macrocysts regressed during treatment. None of the girls' ovaries had a polycystic appearance during or after treatment with the gonadotropin-releasing hormone analogue. Our results confirmed pelvic ultrasonography as a reliable tool for investigation of internal genitalia in girls with precocious puberty and as a valid method for evaluation of the efficacy of treatment with gonadotropin-releasing hormone analogues. We suggest that repeated investigations be performed when evaluating treatment because the morphologic changes, including follicular maturation or regression, reflect ovarian stimulation or suppression. We found no evidence that girls with precocious puberty treated with long-acting gonadotropin-releasing hormone analogues have enlarged polycystic ovaries develop.