Critically ill patients routinely receive H(2) antagonists for stress ulcer prophylaxis while at risk for gastrointestinal bleeding. In these patients it is often difficult to assess accurately the cause of adverse effects such as thrombocytopenia. We evaluate the literature to better define thrombocytopenia related to H(2) antagonist administration and discuss mechanism, potential as a risk factor and case reports describing the severity and duration of thrombocytopenia.