To what extent the standard preparation for sigmoidoscopy (phosphosoda enemas) makes the bowel safe for electrocautery is unknown. Sixty patients were prospectively evaluated to compare the presence of the combustible gases hydrogen and methane during colonoscopy and flexible sigmoidoscopy. Thirty patients underwent flexible sigmoidoscopy after phosphosoda enema preparation, and 30 patients underwent colonoscopy after a polyethylene glycol solution preparation. During colonoscopy, the concentrations of hydrogen and methane remained below combustible levels in all patients. Even segments of colon with significant fecal matter present did not have combustible levels of these two gases. However, at flexible sigmoidoscopy, combustible levels of hydrogen and methane were measured in 3 of 30 (10%) patients. Due to the risk of explosion, electrocautery should not be performed during routine flexible sigmoidoscopy after the standard phosphosoda enema preparation.