OBJECTIVE To determine whether bacterial stool cultures (BSC) are useful in initial evaluation of children with symptoms of nosocomial diarrhea. To answer this question we performed a retrospective record review to determine the yield of BSC in children who developed diarrhea after the third hospital day (HD-3). METHODS The hospital computer record keeping system was utilized to compile the result of BSC collected from children and adolescents ages 0 to 20 years between January 1, 1988, and October 31, 1996. All specimens were analyzed for Salmonella, Shigella, Yersinia and Campylobacter. We reviewed hospital charts of all children who developed a positive BSC beyond HD-3 to determine the time of onset of diarrhea and clinical circumstances. RESULTS A total of 11 516 BSCs were submitted from 9262 children during the 8 1/2-year period. Five hundred sixty-eight (6.6%) of 9262 children had at least 1 positive BSC. Two thousand five hundred seventy-two children had the first BSC submitted after HD-3 and 13 (0.5%) of these children had a positive result. Chart review of these 13 children demonstrated that 6 had onset of diarrhea during the first 3 hospital days. Therefore only 7 children met our criteria for having nosocomially acquired diarrhea caused by a bacterial pathogen. Children whose first BSC was submitted after HD-3 accounted for 3767 (46%) of the total 8126 inpatient BSCs and in excess of $21000 annually in patient billing charges. CONCLUSIONS In the absence of a known exposure the isolation of a bacterial pathogen from the stool of children with onset of diarrhea beyond HD-3 is a rare event. Under most circumstances BSC should not be part of the initial evaluation of children with symptoms of nosocomial diarrhea.