BACKGROUND: The pattern of growth of the uterus was examined by ultrasound examinations of 358 girls who attended a paediatric endocrine outpatient department but were shown not to have any endocrine defect. METHOD: The uterus was measured in length and width at the cervix and at the fundus (cm). Endometrial thickness was measured (mm). Scans were divided by Tanner breast stage and the dimensions compared by one way analysis of variance (ANOVA, with the Student Newman Keuls post hoc test). RESULTS: There was an increase in uterine length, diameter of the fundus, and endometrial thickness at each breast stage from 1 to 5 (ANOVA, p < 0.05), and in the diameter of the cervix with each breast stage from 1 to 4 (ANOVA, p < 0.05). The ratio of the fundus to the cervix increased from 0.95 to 1.29 between breast stages 1 and 4. CONCLUSION: The onset of puberty is marked by an increase in the dimensions of the uterus and in endometrial thickness, but also by a change in the shape of the uterus from a tubular to a pear shaped organ.