Prophylaxis of acute upper gastrointestinal bleeding by control of gastric pH has been widely advocated for intensive care patients. H2-blockers and antacids have been used and demonstrated to be incompletely effective at maintaining gastric pH above 4. A study of 100 patients measured the efficacy of two-hourly gastric pH measurement and titrated therapy consisting of five levels: 1. no therapy 2. ranitidine 50 mg 8 hourly intravenously 3. ranitidine plus Mylanta 30 ml 2 hourly by nasogastric tube 4. ranitidine plus Mylanta 60 ml 2 hourly and 5. ranitidine 100 mg 8 hourly intravenously plus Mylanta II 60 ml 2 hourly. The level of treatment required by proportions of the total study group were (1) 15%, (2) 71%, (3) 96%, (4) 100%. Head-injured and intubated patients generally fell in the more resistant group while patients having had major elective surgery required lower levels of therapy. If control of gastric pH is to be uniformly achieved, a technique of titrated therapy based on gastric pH measurements is supported as cheaper and more effective than other standardised treatment regimens.