In this open study the efficacy and tolerability of rufloxacin in a single dose of 400 mg the first day and 200 mg the nine consecutive days was studied in 26 patients with an acute exacerbation of chronic bronchitis. Twenty-two patients were evaluable for efficacy. Four patients stopped treatment prematurely after five days because of clinical cure. At the enrollment visit a pathogen was isolated in the sputum sample in 19 of 22 evaluable patients. The predominant pathogens were Streptococcus pneumoniae and Moraxella catarrhalis. In 17 of these 19 bacteriologically evaluable patients the initial infecting organism was eradicated from specimens obtained within 48 hours after the end of therapy. There was one case of persistent infection caused by S. pneumoniae (MIC 4 mg/l), one patient had a superinfection with Serratia marcescens (MIC 1 mg/l) susceptible to rufloxacin and therapy was stopped after five days due to clinical failure. One week after the end of therapy, 15 patients remained free from infection whilst one patient experienced reinfection with Klebsiella pneumoniae (MIC 0.5 mg/l). Clinical cure or improvement was observed in 21 of 22 patients. Mild adverse events were reported by two of 26 enrolled patients. In one patient, complaining of headache and dizziness, the adverse events were considered possibly study drug related. No abnormal laboratory findings were reported. Nadir plasma levels of rufloxacin were measured and no accumulation in plasma was observed during treatment. A ten day course of an oral single dose of rufloxacin proved efficacious and was well tolerated in patients with an acute exacerbation of chronic bronchitis.(ABSTRACT TRUNCATED AT 250 WORDS)