The hormonal profile and sonographical assessing of the number and size of the follicles are important in hyperstimulated cycles. Follicular imaging obtained by abdominal scanning may be distorted by echoes from the intestine or by unfavourable location of the ovaries and patients must have a full bladder. We compared the number and size of the follicles at abdominal sonography with the results obtained by vaginal sonography in 37 patients. Vaginal sonography showed more follicles than abdominal sonography because of improved imaging of small follicles on early days of cycle. This might help one to tailor the hyperstimulation to an individual's endocrine response.