A quality improvement project in an academic practice demonstrated a reduction in antibiotic prescribing for acute bronchitis. However, it was unclear whether this represented a reduction in antibiotic use or whether physicians assigned new diagnoses to the same patients to avoid scrutiny and continue to use antibiotic therapy. To examine whether a substantial amount of diagnostic shifting occurred while antibiotic prescribing for acute bronchitis decreased during a 14-month period (from January 1, 1996, to February 28, 1997). All patient diagnoses of acute bronchitis, acute sinusitis, upper respiratory tract infection, and pneumonia were determined for the 14 months of the acute bronchitis intervention. The relative distribution of patients among these 4 diagnostic categories was compared to determine if the percentage of patients with acute bronchitis decreased while those with acute sinusitis and pneumonia increased during the acute bronchitis intervention. The percentage of patients with the diagnosis of acute bronchitis remained unchanged during the 14-month period while antibiotic use for this condition decreased from 66% of cases to less than 21% of cases. Instead of the patients being assigned a different diagnosis such as acute sinusitis so that antibiotic prescribing would not be scrutinized, as we hypothesized, the relative number of diagnoses for acute sinusitis compared with acute bronchitis actually declined during the 14 months. No change was noted in the relative frequency of acute bronchitis cases compared with pneumonia cases. During a 14-month period when an intervention was successful at reducing antibiotic use for acute bronchitis, there was no evidence that physicians shifted patients from the diagnosis of acute bronchitis to other diagnoses.