Diarrhea is a very common event after transplantation, but its cause may be difficult to identify. The first step in determining the cause in any particular case is an understanding of the etiology of diarrhea in general. Although diarrhea often is categorized into such types as secretory versus osmotic, or electrolyte transport-related versus motility-related, a thorough understanding of the problem requires knowledge of how the paracrine, immune, nervous and endocrine systems react to each other as well as to infection, drugs or other stimuli.