Endoscopic measurement of lesions is of great importance in the design and performance of clinical trials, as, for example, in studies of ulcer disease. Endoscopes are constructed with wide-angle lenses that significantly distort the image by creating a relative compression of points in its periphery. We have recently developed a computer program to correct the distortion of the wide-angle lens. We sought to determine the accuracy of the currently used open-biopsy forceps measurement technique and compare it to that of an image-processing technique designed to correct image distortion. The overall error of the open-biopsy forceps technique using an in vitro ulcer model was under-estimation of lesion size by 41.8% +/- 23.3%. When image processing was used to correct distortion, error was significantly decreased to 1.8% +/- 2.2% (p < 0.05). In vivo measurements were made using an inserted object of known size (coated chewing gum). The mean error of the forceps technique in vivo was 26.5% +/- 5.7% (under-estimation of size), which improved significantly to an error of 2.8% +/- 3.2% (p < 0.05) with the image-processing technique. We conclude that image processing significantly enhances the accuracy of measurement at endoscopy.