Gastroenterological medicine has, in recent decades, experienced two major innovations, viz. fibre optic endoscopy (a diagnostic technology) and cimetidine, an innovation in ulcer therapy. This paper is concerned with determining the efficacy of cimetidine in reducing the number of surgical procedures for gastric and duodenal ulcer. It is found that, since the introduction of cimetidine, a statistically significant decline in gastric ulcer operations has occurred. A similar result was not obtained for persons with a diagnosis of duodenal ulcer. The picture of substitution of therapies given by this study is in sharp contrast to that depicted in clinical drug trials. This has significance for technology assessment.