Background: The gold standard for the diagnosis of chronic obstructive pulmonary disease (COPD) is spirometry, but there are barriers to its use in primary care. Aims: To externally validate the COPD Diagnostic Questionnaire (CDQ) as a diagnostic tool in patients at increased risk in Australian general practice and to compare its performance with other CDQ validation studies. Methods: Patients were recruited from 36 general practices in Sydney, Australia. Former or current smokers aged 40-85 years with no prior COPD diagnosis were invited to a case-finding appointment with the practice nurse. The CDQ was collected and pre- and post-bronchodilator spirometry was performed. Cases for whom complete CDQ data were present and the spirometry met quality standards were analysed. Results: Of 1,631 patients who attended case-finding recruitment, 1,054 (65%) could be analysed. Spirometry showed 13% had COPD. The ability of the CDQ to discriminate between patients with and without COPD was fair, represented by the area under the receiver operating characteristic curve of 0.713. With a CDQ cut-off point value of 16.5 the sensitivity was 80% and specificity 47% and, at a cut-off point value of 19.5, the sensitivity was 63% and specificity 70%. Conclusions: The CDQ did not discriminate between patients with and without COPD accurately enough to use as a diagnostic tool in patients at increased risk of COPD in Australian general practice. Further research is needed on the value of the CDQ as a tool for selecting patients for spirometry.