A retrospective study was performed to assess the therapeutic efficacy of cimetidine and antacid, alone and in combination, for the control of gastric acidity and bleeding in pediatric burn patients. The frequency of pH values no greater than 3.5 was significantly greater in the cimetidine group than in the antacid group, and the combined regimen did not enhance the effectiveness of therapy. In all groups, pH values no greater than 3.5 were associated with a significantly higher incidence of positive guaiac tests. In four patients studied prospectively, 63% of the administered dose was detected in the urine 8 h later, compared to the previously reported 24-h delay for normal, healthy adults. Rapid clearance of cimetidine might explain the decreased efficacy of this drug in burn patients. Dosage and schedules for antacid prophylaxis in pediatric burn patients require further study.