To assess the morphological findings based on transvaginal ultrasound, the ovarian volume and the number and size of small follicles were studied in 47 patients with polycystic ovarian syndrome (PCOS) diagnosed clinically. Thirty normal volunteers served as controls. The mean ovarian volume (10.3 ml) and number of follicles (10.6) in patietns with PCOS were significantly higher than those of controls. An ovarian volume > 6.2 ml and follicles > 10 with a diameter of 2–8 mm were the most prominent features in the ovaries of patients with PCOS and 94% of 47 patients with PCOS had at least one of these ultrasonic findings. However, marked asymmetry of the two ovaries was seen in about 50% of patients with PCOS and a considerable overlap existed between controls and patients with PCOS in the ovarian volume and in the number of follicles. Three (6%) patients with normal ovarian volume and normal number of follicles were observed. Therefore, it is difficult to clearly distinguish a patient with PCOS from a normal woman only by transvaginal ultrasonic criteria using the ovarian volume and/or the number of follicles, although these criteria could be clinically useful for the screening of PCOS.