An open, non-comparative clinical trial of cefaclor in adults with acute exacerbations of chronic bronchitis (54 patients) or pneumonia (24 patients) is reported. The dosage of cefaclor used was either 250 mg or 500 mg taken orally three times daily. Clinical cure was obtained in 39 of 42 (93%) of patients on the lower dose and 32 of 33 (97%) on the higher dose. Side effects were minimal and the antibiotic was very well tolerated. Microbiological evaluation was possible in 32 of 75 (43%) of patients in whom potential pathogens were identified before treatment. Microbiological 'cure' was achieved in 17 of 32 (53%), the majority of whom had received the higher dose. The lack of correlation between the clinical and microbiological results cast further doubts on the value of the standard sputum culture methods in the diagnosis and management of lower respiratory tract infection. Cefaclor is useful in the management of acute lower respiratory tract infections by virtue of its excellent clinical efficacy and safety.