<b><i>Background:</i></b> Recruiting patients for research in primary care is difficult with diseases that tend to remain underdiagnosed, such as chronic obstructive pulmonary disease (COPD). Researchers may consider introducing case finding into patient recruitment, but the impact on recruitment yield is largely unknown. <b><i>Objectives:</i></b> To assess the impact of case finding on recruitment yield and population characteristics in primary care-based COPD research. <b><i>Methods:</i></b> For a cluster randomized controlled trial of COPD in primary care, an opportunistic case finding strategy was introduced into patient recruitment, in addition to recruiting patients with previously diagnosed COPD. The recruitment process and performance of the primary care physicians (PCPs) were analysed. The numbers and characteristics of patients identified by case finding were compared with those of patients with previously diagnosed COPD. <b><i>Results:</i></b> Thirty-five PCPs approached 398 patients and successfully recruited 216 patients during 1 year. The mean number of patients recruited was 6.3 (range 0-16) per PCP. Case finding contributed 71 patients (32.9%) with significantly milder disease, with FEV₁ % predicted +16.7 (95% CI: +11.3 to +22.0), a COPD Assessment Test difference of -4 points (95% CI: -2 to -6; p < 0.001), and fewer exacerbations resulting in a higher rate of GOLD class A (85.9 vs. 45.5%; p < 0.001). The smoking rate was significantly higher among patients with newly diagnosed COPD (70.4 vs. 48.6%; p = 0.002). <b><i>Conclusion:</i></b> Case finding increased the number of patients recruited by 50%. The COPD patients identified by case finding differed importantly from those with previously diagnosed COPD. Researchers should be aware of COPD underdiagnosis and the potential impact of case finding during patient recruitment.