The frequency and consistency of stools of all patients at a VA Medical Center who were tube-fed during a 3-mo period were recorded prospectively and analyzed in terms of eight definitions of diarrhea derived from the literature. The extent of diarrhea, reported as incidence and as percentage of days with diarrhea, was used to determine differences among the definitions. The relationship between extent of diarrhea and duration of monitoring patients was also determined. Results of 29 patients monitored for 13.0 d (6.5 d) [median (interquartile range)] indicated that the definition of diarrhea significantly influenced the reported incidence of and percentage of days with diarrhea. Duration of monitoring showed a significant, positive relationship to the incidence of diarrhea (ie, the longer the duration, the more likely that diarrhea was observed). When diarrhea was reported as the percentage of days with diarrhea, the influence of monitoring duration virtually disappeared.