The subjects were 241 patients with lower respiratory tract infections treated at two medical centers in Austria; 161 of the patients were evaluated clinically and, of these, 69 were evaluated bacteriologically. The patients were randomly assigned to receive ofloxacin (n = 101) or a comparison drug (doxycycline [n = 27] or amoxicillin-clavulanate [n = 33]). The mean age of patients in the ofloxacin group was 61 years; that of patients in the comparison groups was 64 years. The mean duration of treatment was nine days; most patients received the study drug intravenously for three days and then orally for four to seven days. In most patients, the doses were 200 mg of ofloxacin twice daily; 100 mg of doxycycline twice daily; or 2,200 mg of amoxicillin-clavulanate intravenously two or three times daily, followed by 625 mg orally two to four times daily. The clinical response was rated satisfactory (all pretreatment symptoms disappeared) in 28% of the ofloxacin-treated patients and in 22% of the other two groups; improved (all except one symptom improved) in 70% and 65%, respectively; and unsatisfactory in 2% and 13%, respectively. The bacteriologic response was rated satisfactory in 59% of the 39 evaluable ofloxacin-treated patients and in 50% of the 30 patients in the other two groups. Most of the unsatisfactory responses were associated with persistent strains of Streptococcus pneumoniae. Transient side effects of mild to moderate severity were noted in nine of the 101 ofloxacin-treated patients and in seven of the 60 patients in the other two groups.(ABSTRACT TRUNCATED AT 250 WORDS)