AIM: To search the pathophysiological mechanism of diarrhea based on daily stool weights, fecal electrolytes,osmotic gap and pH.METHODS: Seventy-six patients were included: 51 with microscopic colitis (MC) [40 with lymphocytic colitis (LC);11 with collagenous colitis (CC)]; 7 with MC without diarrhea and 18 as a control group (CG). They collected stool for 3 d. Sodium and potassium concentration were determined by flame photometry and chloride concentration by titration method of Schales. Fecal osmotic gap was calculated from the difference of osmolarity of fecal fluid and double sum of sodium and potassium concentration.RESULTS: Fecal fluid sodium concentration was significantly increased in LC 58.11±5.38 mmol/L (P＜0.01)and CC 54.14±8.42 mmol/L (P＜0.05) than in CG 34.28±2.98 mmol/L. Potassium concentration in LC 74.65±5.29 mmol/L (P＜0.01) and CC 75.53±8.78 mmol/L (P＜0.05) was significantly less compared to CG 92.67±2.99 mmol/L.Chloride concentration in CC 36.07±7.29 mmol/L was significantly higher than in CG 24.11±2.05 mmol/L (P＜0.05).Forty-four (86.7%) patients had a secretory diarrhea compared to fecal osmotic gap. Seven (13.3%) patients had osmotic diarrhea.CONCLUSION: Diarrhea in MC mostly belongs to the secretory type. The major pathophysiological mechanism in LC could be explained by a decrease of active sodium absorption. In CC, decreased Cl/HCO3 exchange rate and increased chloride secretion are coexistent pathways.