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