This article is a review of the causative factors and pharmacologic treatments of diarrhea. This information was incorporated into a Diarrhea Assessment and Treatment Tool (DATT) to guide clinicians on comprehensive diarrhea assessment and current treatment recommendations. The tool was utilized at a university-affiliated oncology institution by a clinical nurse specialist on 26 patients as a performance improvement project. Ease of use and efficacy of DATT were tested. Eighty-one percent of patients were assessed using DATT in 30 minutes or less. Seventy-nine percent of the 57 identified diarrhea classifications were not being treated upon initial assessment. Diarrhea control was achieved in 73% of the patients within 7 days or fewer when DATT was utilized. The premise of diarrhea management is that if all the causative factors are not treated, diarrhea will persist. The conclusions are that this tool will aid the clinician in a comprehensive assessment of diarrhea and provide a systematic approach to diarrhea treatment. The need for research on best practice for management of the various causative factors of diarrhea is needed.