The colonic concentrations of the combustible gases, H2 and CH4, were well below hazardous levels in 60 consecutive patients at the time of colonoscopy. Independent analysis of the effect of a low residue liquid diet, a 12-hr fast, and bowel cleansing on the pulmonary excretion of these gases suggest that the low colonic concentrations encountered were largely the result of our patient preparation procedure. Both ingestion of the liquid diet and fasting decreased the pulmonary excretion of H2 markedly but caused only a slight fall in CH4 excretion. Thus H2, but not CH4, production appears to depend on the delivery to the colonic bacteria of exogenous fermentable substrate. Bulk removal of bacteria from the colon resulted in about a 10-fold reduction in the excretion of both gases. The results of these studies do not support the need for routine CO2 insufflation prior to colonoscopic electrosurgical polypectomy.