The role of bacterial infections in chronic obstructive pulmonary diseases is still poorly understood, and so is the importance of antimicrobial chemotherapy. Based on the present knowledge from clinical studies, it is suggested that antibiotic treatment should be initiated for seven to ten days in patients suffering from an 'Anthonisen type-1 exacerbation', i.e. clinical deterioration with a significant increase in sputum production and sputum purulence, but not in others. The best tolerated, least toxic and least expensive oral antibiotic with adequate activity against pneumococci and H. influenzae is probably most appropriate. These prerequisites are still best fit by aminopenicillins and cotrimoxazol.