In 60 cases of hyper-aggressive duodenal ulcer an endoscopic examination was made, at short intervals (4,7,11,13,21,28 days) after the treatment with H2-receptor antagonists. The open trial was based on random cases under ranitidine 150 mg b.i.d. (31 cases) or cimetidine 400 mg b.i.d. (29 cases). The healing rate has been calculated from the regression slope of the ulcerative area in mm2. Regression does not have a linear slope; it is slow in the first week, increases in the second week, in which it reaches the maximum value, and then decreases gradually in the following weeks. H2-receptor antagonists have no doubt an accelerating effect on healing; cimetidine, in comparison with ranitidine, has a faster and more evident action in the first week. But then, it reduces its action more rapidly in the following weeks so that, on the whole, it turns out to be slightly less valuable. These results can be practically useful for the treatment of hyper-aggressive D.U. as far as early risks are concerned.