The influence of patient relevant parameters such as age, comorbidity, or duration of disease on the treatment success of levofloxacin for community-acquired respiratory tract infections (CARTI) has not been thoroughly elucidated. We therefore conducted a prospective cohort study of 9831 patients with CARTI in a clinical practice setting. The patients received 500 mg of levofloxacin once a day over a mean of seven days. Twenty-two attributes per patient were recorded before treatment initiation and after seven to fourteen days after start of treatment. Descriptive and explorative statistics such as the k-means and C4.5 algorithms were used to analyze the dataset. The overall success rate of levofloxacin therapy for CARTI was over 98%, side effects occurred in 1.6% of patients. Descriptive analysis revealed a weak correlation between parameters which significantly influence the course of disease, such as the number of comorbidities, the duration of infection before levofloxacin start, or the severity of symptoms and the treatment success. Explorative statistics yielded similar results. Two homogenous clusters, holding 34 and 45% of patients respectively, yielded the number of comorbid conditions and the duration of infection as main attributes negatively influencing treatment success. We therefore conclude, that the number of co-morbid conditions and the duration of infection before start of treatment as the strongest negative predictors for treatment success.