A meta-analysis was performed of 15 randomized studies on the prophylaxis with cimetidine and/or ant-acids of upper GI bleeding acquired in the ICU. There were eight comparisons of a group receiving cimetidine with a control group, nine comparisons of a group receiving antacids with a control group, and ten comparisons of a group receiving cimetidine with a group receiving antacids. The incidence of upper GI bleeding ranged from 3.4% to 52.7% among 866 control patients who received either a placebo or no prophylaxis. In five of eight comparisons, cimetidine was significantly more effective than no treatment or a placebo to prevent occult and overt upper GI bleeding; the typical odds ratio was 0.32 (95% confidence interval 0.21 to 0.49). In six of nine comparisons, antacids were significantly more effective than no treatment or a placebo; the typical odds ratio was 0.12 (0.08 to 0.19). Finally, antacids were significantly more effective than cimetidine in two of ten comparisons; the typical odds ratio was 1.61 (0.97 to 2.65). However, weaknesses in the study designs, heterogeneity of treatment effects, the lack of strength of the accumulated evidence, and the fact that no utility has been shown in terms of reducing morbidity (shock, need for transfusion) or mortality, prevent any definitive conclusion in regard to compulsory use of upper GI bleeding prophylaxis for ICU patients.