A total of 204 patients between the ages of 20 and 80 and suffering from respiratory tract infections were treated by general practitioners with doxycycline. The infections were representative of those usually encountered in practice except that there were only three cases of pneumonia. The response to treatment, both subjectively and objectively, was good; by the third day, 84% of patients were improved. 89% of patients were cured by the tenth day. Failure to improve was noted in only 5% and failure of cure in 11% of patients. The participating physicians assessed the result of treatment as good or very good in 87% of patients and as moderate in 8%, only 5% of the patients had a poor or negative result.