An investigation into a mild diarrhea in a group of grower/finisher pigs was carried out in order to determine the etiology. A tiamulin injection and a carbadox-medicated ration were given to pens of pigs in a 2 x 2 factorial experimental design. Pens of pigs were assessed a score, based on the consistency of the feces in the pen, each week. The clinical investigation looked for the intestinal pathogens Brachyspira pilosicoli, B. hyodysenteriae, Lawsonia intracellularis, Salmonella spp., Yersinia spp., transmissible gastroenteritis virus, and rotavirus. Despite a rigorous investigation, the diarrhea was not attributed to any pathogen. A mild colitis was noted among pigs necropsied while affected with diarrhea. Improved diagnostic tools may allow a more effective response to an outbreak of mild disease, while at the same time reducing the amount of antimicrobials used in swine production.