Background: Amoxycillin/clavulanate is considered fi rst-line treatment for ambulatory exacerbations of COPD. However, narrow-spectrum antibiotics may be as useful for mild to moderate patients. Objective: To compare the clinical effi cacy of amoxycillin versus amoxicyllin/clavulanate in exacerbations of COPD in primary care. Methods: A randomized, double-blind, noninferiority clinical trial was carried out in eight primary care centers in Catalonia, Spain. Spirometrically-diagnosed patients older than 40 years with COPD, without criteria of hospitalization and Anthonisen's types I or II exacerbations were included. The main outcome was clinical cure at the end of treatment (EOT) visit on day 10. Results: A total of 137 patients were enrolled in the study (68 assigned to amoxycillin and 69 to amoxycillin/clavulanate). The mean forced expiratory fl ow in one second was 61.6% and the mean age was 71.4 years. At EOT, 92.8% of patients in the amoxycillin/clavulanate and 90.9% in the amoxycillin group were considered clinically cured, a statistically non-signifi cant difference. Adverse effects were observed in 11 subjects, 3 in the amoxycillin group and 8 in the amoxycillin/clavulanate group, 2 of whom required a change in treatment. Conclusions: Amoxycillin was at least as effective clinically and as safe as amoxycilin/ clavulanate in the treatment of acute exacerbations of COPD in mild to moderate patients in primary care.