We prospectively studied 26 patients receiving simethicone (n = 14) or placebo (n = 12) in Colyte (Edlaw Preparations. Inc., Farmingdale, NY, U.S.A.) to determine if the addition of simethicone could improve visibility when administered the night before colonoscopy. Two parameters of visibility were assessed: amount of bubbles and degree of haziness. The effectiveness of a night-prior administration of Colyte to clean the colon of feculent debris was also examined. Patients receiving simethicone had significantly less bubbles (p less than 0.02), but haziness was not improved (p = 0.9). The presence of feculent debris that interfered with the colonoscopic examination was similar in both groups: simethicone 5 of 14 or 35% and placebo 7 of 12 or 58%. Eighty percent (four of five) of patients with feculent debris in the rectosigmoid colon had diverticulosis, and 50% (four of eight) patients with diverticulosis had feculent debris in the rectosigmoid. These data indicate that (a) the combination of simethicone plus Colyte administered the night before colonoscopy improves visibility by diminishing bubbles; (b) this dosage of simethicone is not effective in diminishing haziness when administered the night before colonoscopy; and (c) patients with diverticulosis are likely to have feculent debris in the rectosigmoid colon, and a precolonoscopy enema may be helpful when the diagnosis is known.