Controversy exists over the diagnosis of idiopathic small intestinal bacterial overgrowth (SIBO) in dogs and some clinicians use the term antibiotic-responsive diarrhea (ARD) in preference. However, whether such terms are interchangeable is not clear. To examine the relationship between duodenal bacterial numbers and a clinical response to antibiotics, SIBO and ARD were defined by nonoverlapping criteria. Quantitative duodenal juice bacteriology and indirect serum biochemical tests were used to assess small intestinal bacterial populations in 30 dogs with gastrointestinal disorders, including 9 with ARD. Serum total unconjugated bile acid (TUBA) concentrations were measured in all dogs, serum folate and cobalamin concentrations were measured in 29 of 30 dogs, and quantitative culture of duodenal juice was performed in 22 of 30 dogs. Serum TUBA concentrations also were measured in samples from 38 control dogs. Twenty of 22 affected (clinical) dogs in which quantitative bacteriology was performed were classified as having SIBO (>105 colony-forming units of total bacteria per milliliter of duodenal juice), but bacterial numbers did not differ significantly between dogs with ARD and dogs with other disorders. Increased folate (19/29), decreased cobalamin (16/ 29), or a combination (9/29) were common, but increased TUBA concentrations were documented in only 5 of 30 clinical dogs. Again, no significant differences were observed between dogs with ARD and those with other disorders, and a similar proportion (5/38) of controls had abnormally high TUBA concentrations. Finally, no significant differences were noted when duodenal bacteriology and TUBA concentrations were assessed before and during antibiotic therapy. These results question the utility of quantitative duodenal juice bacteriology and indirect biochemical marker tests for SIBO in the investigation of canine gastrointestinal disorders.