BACKGROUND: Diarrhea is frequently seen in developed and developing countries, and severe diarrhea is characterized by the high risk of death. Thus, it is very important to assess the severity of diarrhea early. We conducted a multi-center study to identify risk factors for the severity of diarrhea in adult patients and formulate an adult diarrhea state score (ADSS) for out-patient clinicians. METHODS: A total of 219 adult patients with acute diarrhea were divided into two groups: 132 patients with mild diarrhea and 87 with severe diarrhea. Logistic regression was used to determine risk factors for the severity of diarrhea. The risk factors were assessed and an ADSS was formulated. Receiver operating characteristic (ROC) analysis was made to evaluate the diagnostic accuracy of ADSS, and the Kappa test was used to confirm the diagnostic reliability. RESULTS: Five risk factors for evaluating the severity of diarrhea in adults included age (P<0.05), axillary temperature (P<0.01), mean arterial pressure (P<0.01), white blood cell count (WBC; P<0.01), and WBC in stool (P<0.01). The area under the ROC curve for ADSS was 0.958 when the cut off value was 4 (a sensitivity of 0.909; a specificity of 0.874), and the Kappa value was 0.781 (P<0.05). CONCLUSION: The risk factors associated with the pathogenic condition of diarrhea were identified, quantified and formulated into an ADSS, which has high diagnostic accuracy and reliability for the early identification of patients with severe acute diarrhea.