Oxmetidine is an H2-antagonist like cimetidine containing an imidazole ring in its molecule, but differing from cimetidine in that it contains in the side-chain a substituted isocytosine moiety in place of the cyanoguanidine group. Nine controlled clinical trials are critically analysed in detail. The overall results show that the antiulcer activity of oxmetidine is not significantly different from that of cimetidine with mean healing rates by week 4 of 74.9% and 75.3%, respectively. Healing rates proved to be lower in smokers than in non-smokers in all trials but one. Satisfactory response as regards symptoms was obtained both with oxmetidine and cimetidine. A certain degree of variability with regard to the untoward effects was found, but in all cases failed to prove significant from the clinical point of view. However, a better definition of oxmetidine safety requires a study on a large number of patients and for a longer period.