Since the published incidence of tube feeding-associated diarrhea varies widely between 2.3 and 68%, and because commonly used definitions of diarrhea are subjective and imprecise, we determined the actual fecal output of malnourished patients during the first 6 days of tube feeding. Nine patients on the same isoosmolar formula were evaluated for diarrhea. Patients were interviewed daily as to whether they thought diarrhea was present or absent. Frequency of defecation was recorded, and stool was collected daily for weight. When assessed subjectively, the incidence of diarrhea rose from 11% on day 1, to 25% on day 4, and to 43% by day 6. The group (n = 4) complaining of diarrhea averaged 3.0 bowel movements per day, whereas those denying diarrhea (n = 5) averaged fewer than 0.2 bowel movements per day (p less than 0.01). Stool weight, however, plateaued for all patients by day 6 at 52.7 +/- 35.7 g of stool per day. No patient produced in excess of 250 g of stool per day. We suggest that traditional subjective measures for assessing diarrhea may be flawed and should not be relied upon when evaluating and treating tube feeding-associated diarrhea.