As a result of the magnification and distortion caused by the fish-eye lens of the panoramic hysteroscope, the size of images cannot be measured accurately. Magnification can alter the apparent size of images by up to 27%, while lens distortion can produce an alteration of up to 28%. These effects are independent and may be additive. The present study applied a new technique for measuring the diameter of intrauterine structures at hysteroscopy to measurement of the superficial vasculature in 34 Norplant users recruited from a family planning clinic in London, England, and in 20 controls diagnosed at the hysteroscopy clinic with ovulatory dysfunctional uterine bleeding. This technique involved advancement of biopsy forceps into the field of view, use of a grid of known internal size, and computer-assisted transformation of the curved image into a flattened one. The mean diameter of superficial dilated vessels in Norplant users (120 mcm) exceeded that in women with menorrhagia (74 mcm). Scattered dilated vessels up to 777 mcm in diameter were observed on the endometrial surface in some Norplant users. The small endometrial polyps observed in Norplant users appeared to have a single fine vessel in the pedicle and were transparent. The image correction technique reduced the effect of image magnification on the apparent size of an object viewed through the hysteroscope to 7% and the effect of distortion to 3%.