Brodimoprim and doxycycline monotherapies were compared in the treatment of acute respiratory tract infections in a randomized double-blind trial. The brodimoprim dose was 400 mg as a single loading dose followed by 200 mg for 10 days at bedtime. The corresponding doses of doxycycline were 200 and 100 mg respectively. Both treatment groups consisted of 30 patients. In the brodimoprim group, 23 patients recovered completely, five improved and two did not respond. In the doxycycline group, 26 patients recovered completely, three improved and one was considered a failure. Only one patient in the brodimoprim group had moderate diarrhoea.