It has been established that long-term low-dose erythromycin therapy (EM therapy) is very effective for sinobronchial syndrome, a common condition in Japan characterized by chronic upper and lower airway inflammation. The effect does not result from its bacteriocidal activity and the detailed mechanisms are not known. It takes 3-6 months for EM therapy to improve the symptoms. This study was designed to evaluate the additive effect of continuous low dosage or intermittent usual dosage of ofloxacin (OFLX) on EM therapy in patients with sinobronchial syndrome. Patients with sinobronchial syndrome were randomly allocated to receive one of the following four regimens. Patients in Group A received both low-dose OFLX and EM therapy daily for 6 months. Patients in Group B received EM therapy and intermittent treatment of OFLX for 6 months. Patients in Group C underwent EM therapy for 6 months. Patients in Group D received neither OFLX nor EM therapy. All patients were given carbocystein for more than 2 months before starting each treatment and during the study period. In patients receiving OFLX and/or EM therapy, these antimicrobial agents were well-tolerated during the treatment period. Amount of sputum in the morning was significantly less in Group C than in Group D after 3-6 months, and decreased significantly in Group A as compared with Group B after 2 weeks, Group C after 2 weeks to 2 months, and Group D after 2 weeks to 6 months. Other symptoms such as number of expectorations, difficulty of expectoration and severity of cough also improved rapidly in Group A. These findings suggest that it is useful to add low-dose OFLX to EM therapy for sinobronchial syndrome, especially within 1-2 months from starting treatment, and it may be cost-effective as this combination therapy can shorten the treatment period of EM therapy.