Abstract A survey has been made of the bacteria present in the gastric aspirate obtained from 280 preoperative and postoperative secretion studies. In the preoperative group viable bacteria were isolated from 41 per cent of patients, whereas in the postoperative group they were isolated from 73 per cent. The nature of the bacterial flora also differed in the two groups. It is suggested that infected stomach contents may cause local sepsis following gastric surgery and that loss of the acid barrier may allow infection by pathogenic bacteria of the gut, producing postoperative diarrhoea.