Age-related changes in uterine and ovarian morphology and size were studied in 114 normal premenarcheal girls using a real-time mechanical sector scanner. Reference growth values were obtained for total uterine length, anteroposterior diameters of the corpus and cervix, corpus/cervix ratio, and uterine and ovarian volumes. Up to 7 years of age uterine size was not influenced by age and there was a relative predominance of the cervix over the corpus. Thereafter the uterus underwent a regular increase in size and the corpus gradually became larger than the cervix. Ovarian volume was relatively stable up to 5 years, when regular age-related growth started, associated with an increase in cystic functional changes in the ovary. As uterine and ovarian morphology and size are reliable indicators of the degree of sex-hormone stimulation, real-time ultrasound offers a rapid method of studying normal and abnormal pubertal development in girls.