To study the function of the colon in cholera, 12 patients with acute cholera diarrhea were subjected to measurements of ileocecal flow rates, fecal flow rates, and ionic compositions of stool and ileocecal fluid. Subtraction of fecal flow rates from ileocecal flow rates was taken as a measure of net fluid absorption by the colon. Additionally, these patients underwent colonoscopic perfusion of the colon that measured net colonic absorption rates of water and ions. The mean ileocecal flow rate was 7.9 ml/min compared with a mean fecal flow rate of 7.6 ml/min, indicating a small mean net fluid absorption by the colon of +0.30 ml/min. By colonoscopic perfusion, 6 patients showed net colonic absorption of water and 6 patients net secretion of water with a slight mean net fluid secretion of -0.03 ml/min. The handling of ions by the colon showed mean net absorption of sodium (100 mu Eq/min) and chloride (127 mu Eq/min), and net secretions of potassium (-42 mu Eq/min) and bicarbonate (-112 mu Eq/min). During convalescence, 5 patients who were studied again all showed net colonic absorption of water, and the handling of potassium changed significantly from net secretion in acute disease to net absorption (p less than 0.05). These results showed that the colon contributes to the clinical expression of cholera by failing to absorb water normally, and by secreting potassium at high rates.