Chronic diarrhea was reported among a cluster of residents of the community of Brainerd, Minnesota. Routine enteric laboratory tests revealed no pathogens. Yersinia enterocolitica was later recovered from the stool of one case patient. E. coli O53:H10 was isolated from sixteen stool samples. Results of laboratory studies on a few patients suggested a secretory mechanism for the diarrhea. Some of the case-patients were treated with antibiotics, but none responded to this therapy; fortunately the symptoms did not worsen with treatment. A case-control study showed that drinking raw milk produced by a single dairy was associated with the development of this chronic diarrheal condition. The cultured raw milk samples yielded numerous bacterial organisms, including E. coli O53:H10, but no commonly recognized pathogens were identified. No toxins were detected in the cattle feed mix or silage. The assumption was that an infectious agent caused the diarrhea because a common vehicle was identified, a common incubation period was observed, and three case-patients developed chronic diarrhea after drinking only one glass of milk. Although the rare, E coli serotype, E. coli 053:H10, was isolated more frequently from stools from case-patients than from those from well subjects, the organism was identified in only 25% of the case-patients. No antibody response to E. coli O53:H10 was noted in the case-patients samples. E. coli O53:H10 was found in stools of some community control subjects who drank raw milk and remained well. It seemed likely that the organism was in the raw milk and could colonize the intestine without causing disease. A definitive cause for the diarrheal syndrome could not be determined. The investigators labelled the disease syndrome as Brainerd diarrhea.