['To investigate a possible difference between true ovarian volume and ovarian volume estimated with two-dimensional (2D) transvaginal ultrasound.', 'Prospective clinical and laboratory study.', 'University hospital research laboratory.', 'Premenopausal girls and women from three Danish national fertility centers (A: n = 42; B: n = 6; C: n = 18), who had one entire ovary removed for cryopreservation of ovarian cortex.', 'Transvaginal 2D ultrasound measurement of ovarian volume before oophorectomy. True ovarian volume was obtained by weighing the ovary.', 'Ovarian volume estimated by weight and ultrasound.', 'Ovarian tissue density was 1.00 g/mL. Mean ovarian volume by ultrasound vs. weight in the three groups was as follows: A: 6.3 mL vs. 7.8 mL; B: 5.4 mL vs. 6.8 mL; and C: 2.8 mL vs. 6.1 ml. Ovarian volume obtained by ultrasound was at least 27% smaller than the true ovarian volume.'] Is true ovarian volume underestimated by two-dimensional transvaginal ultrasound measurement?