We examined the risk factors for bacterial exacerbation, defined as the presence of pathogenic bacteria in sputum, in 90 chronic obstructive pulmonary disease (COPD) patients with an exacerbation and changes in sputum characteristics. Smoking, alcohol, lung function, body mass index, medical visits and treatments were the independent variables assessed using multivariable logistic regression modelling (OR, 95% CI). A bacterial exacerbation was diagnosed in 39 (43·3%) of 90 patients. Bacterial exacerbations were more prevalent among current smokers (OR 3·77, 95% CI 1·17–12·12), in patients with poor compliance with inhalation therapy (OR 3·25, 95% CI 1·18–8·93) and with severe lung function impairment (FEV1 OR 0·96, 95% CI 0·93-1·00). Prior use of antibiotics was a risk factor for Pseudomonas aeruginosa infection (OR 6·06, 95% CI 1·29–28·44) and influenza vaccination appeared to have a protective effect against this infection (OR 0·15, 95% CI 0·03–0·67). We conclude that severe impairment of lung function, smoking and poor compliance with therapy are risk factors for bacterial infection in COPD, and P. aeruginosa should be suspected in patients who have been treated with antibiotics and in those not vaccinated against influenza.